"Random" soft brake pedal recovers when pumped

[moss1972]

Hi Folks,

All new calipers pads and rotors 5k miles ago. Bled to success and rebled after 3k. Anyway...the last couple of months I have a random situation with the brakes. I notice it alot when I'm engine braking in 4th. Like down an offramp. Sometimes it happens when engine braking to a stop in lower gears (like 2nd or 3rd). I've tried to replicate it but it's not something I can really nail down. At first I thought it was a vacuum thing...and it may be.

I rebled everything to success and it seemed to go away for a few days...but then again the problem seems random so that bleeding may have done nothing.

What happens is I will go to brake and the pedal will go almost to the floor before the brakes catch. They do catch...but very low. Then if I pump, they are like new (meaning new after replacing pads, rotors, calipers and lines). If I anticipate the problem and just pump the brakes every time I stop then I don't experience the problem.

But it seems very random. I don't know if it's a vacuum issue. Master cylinder plunger going bad? Air moving around somewhere in the system up near the master?

Can anyone give me a ghost hunting direction on this?

[18Fiatsandcounting]

A soft (spongy) pedal all the time is usually a sign of air in the lines or the calipers, but what you're describing sounds more like a master cylinder issue. It could be leaking seals, trapped air in the master cylinder, etc. I assume you've checked the brake fluid level in the reservoir and it's not dropping over time?

I did have a Fiat many years ago that had leaking seals after I had rebuilt the calipers, and you had to pump the brakes to get them firm. Evidently, the seals were either old or not quite right for those calipers. Never did figure it which it was, but a new set of seals fixed that problem. But, since you have new calipers, I'm guessing that's not the problem.

I suggest starting with the master cylinder. The fact that the issue appears "random" also points in the direction of a seal that sometimes holds and sometimes doesn't.

-Bryan

[SteinOnkel]

Have had this experience with every caliper rebuilt kit. As far as I'm concerned, they are simply not rebuildable. Neither is the MC.

The parts are so cheap, I would just replace them.

But before you do that, go over every single connection point in the brake system and make sure they are dry. If you've got a leak somewhere, that would also explain your issue. Might just need to crank down on some union somewhere a bit harder. That's the thing with brake components, when you think they are tight enough they need another half a turn.

Also, run your car at idle (with engine warmed up) and pump the brakes. Does the idle fluctuate and become lumpy? That would mean your booster is on its way out.

Cheers
STeiny

[moss1972]

Thank you Bryan and Steiny.

I was wondering if I should replace the vacuum booster while I was at it but I'm going to go do the MC and hope for the best. I am doing all new hoses, reservoir and MC. Not that the rez or hoses matter....but for $7, may as well.

Not losing any fluid...rez is consistently full. Everything looks dry as well. But excellent points.

I will try the engine idle test...that's a great tip!!!

Thanks fellas. I was leaning toward MC seals on the way out. Part of me was wondering because I notice it more during engine braking if it was a vacuum issue due to the drop in pressure on the intake manifold side "robbing" the vacuum. Could still be. Guess we will do one part at a time.

[18Fiatsandcounting]

Part of me was wondering because I notice it more during engine braking if it was a vacuum issue due to the drop in pressure on the intake manifold side "robbing" the vacuum. Could still be. Guess we will do one part at a time.

There is another thought that I had, although I didn't put it in my first post: You have air in the brake lines, and your brake booster is marginal in terms of how much assist it gives. In other words, if your brake booster isn't working most of the time, then the pedal will feel firmer (although it shouldn't go the floor). When you decelerate while in gear, your intake manifold manifold vacuum is about as high as it will get, so maybe the booster works enough at that point to make it feel like the pedal is easier to press to the floor.

The test for this is easy. With the engine off, press on the brake pedal really hard and hold it. It should remain firm. If the pedal slowly goes to the floor (or moves at all), then fluid is either leaking past the seals in the M/C or past the seals in the calipers. If it takes several inches of downward travel before the pedal feels firm, you have air in the system.

-Bryan

[moss1972]

Part of me was wondering because I notice it more during engine braking if it was a vacuum issue due to the drop in pressure on the intake manifold side "robbing" the vacuum. Could still be. Guess we will do one part at a time.

There is another thought that I had, although I didn't put it in my first post: You have air in the brake lines, and your brake booster is marginal in terms of how much assist it gives. In other words, if your brake booster isn't working most of the time, then the pedal will feel firmer (although it shouldn't go the floor). When you decelerate while in gear, your intake manifold manifold vacuum is about as high as it will get, so maybe the booster works enough at that point to make it feel like the pedal is easier to press to the floor.

The test for this is easy. With the engine off, press on the brake pedal really hard and hold it. It should remain firm. If the pedal slowly goes to the floor (or moves at all), then fluid is either leaking past the seals in the M/C or past the seals in the calipers. If it takes several inches of downward travel before the pedal feels firm, you have air in the system.

-Bryan

I did the adhoc tests. With engine in idle I pump the pedal quickly...it DOES boost engine idle 200 rpms or so until I stop. As suggested above...bad vacuum booster...or going bad?

I did the above adhoc test of pressing pedal hard with engine off. Firms right up and holds. Not the MC or Caliper seals.

I'm flummoxed, honestly. It did it more than usual today. I braked normally to a stop. Was in neutral and still rolling slowly up to the stop sign. Went to hit the brakes and the pedal went to the floor. Just as the pedal hit the floor the brakes caught. I pumped it and they were normal. WTH. So bizarre.

Not losing fluid. All new calipers, pads and most hoses...all proper parts are greased. I bled the brakes again a few weeks ago.

At this point I have a new vacuum booster, MC, rez and rubber feed lines. Gonna do everything this afternoon. So at this point the only thing I have not replaced is the hard lines going to the rear and the front soft lines. Crossing fingers and swearing.

[18Fiatsandcounting]

Steve, that is weird. Taking each item one at at time:

Yes, if the idle goes up when you pump the brakes, that means extra air is getting from the booster into the intake manifold. Extra air in the intake manifold (beyond what the engine is expecting) causes the idle to go up. Going up by 200 rpm isn't bad (I've hear of it going all the way up to 2000 rpm), but since you already have a new booster, might be the time to replace.

On the pedal going to the floor. I recall now that I had a 124 sport coupe many years ago where I had rebuilt the brake system. Everything seemed fine with the brakes, but one day the pedal went to the floor and I coasted into the vehicle in front of me. It was in a parking lot, and I was only going about 5 mph (fortunately), but the brakes just failed. The issue turned out to be that the caliper seals weren't quite right, and so one day they just didn't feel like working. New seals solved the problem. So, although I hate to say it since you bought new calipers, but they might have come with old or really poor quality seals...

-Bryan

[blazingspider]

My 2 cents..

The last time my master cylinder went I had similar symptoms; a sometimes soft pedal. Eventually, the pedal went to the floor when trying to stop and I had to use the e-brake. I never did see any fluid leaks anywhere and the reservoir was always filled. I replaced the master cylinder and all was well after that. I'm assuming here that the internal seals were shot in the mc and couldn't hold any pressure.

You may also have an issue with the brake booster. For years I could pump the brakes and that firmed up the pedal a bit and the idle speed would be affected, but not by much. Eventually the diaphragm in the booster finally went for good and when braking, I heard a very distinctive hissing sound coming from the brake pedal area, the pedal went to the floor and as there was now a huge vaccum leak, the idle speed was affected as well.

I temporarily bypassed the booster until I could replace it. If you don't hear that hissing sound when braking then I would concentrate on replacing the mc first. As others have noted here, replacing the brake booster is a major pain but when it's time, it's time. Braking without the booster is not a very good feeling. You really have to stand on the pedal to get any stopping action without it.

Good luck!!

[moss1972]

Thank you Bryan and Blazingspider. Both of you have been a huge help. Bryan, all your responses led me to simply choosing to replace everything and cut out all chances of missing the actual problem.

Blazingspider...It sounds like we lived similar tales. I'm glad I replaced everything because I felt in my gut this was not headed down a road I wanted to travel (hahaha). On Monday, the day I started the job...the pedal went soft 3 times in one short trip. It was only a matter of time.

Now I'm going to put this up here for posterity and as a resource...and as thanks.

As I found out...replacing the booster can be a geometry equation that involves a lot of swearing. But after removing the choke off the driver's side of the carb it was able to slide out. Well, scrape and tug out. Going in was just as fun, don't despair! But with care and planning it IS dooable and pretty easy, really. 4 nuts on the vacuum, two on the master...two hoses and 3 brake lines.

I emptied the res then disconnected the hoses from it and removed the bracket to make room. Said a prayer to all the gods and removed the brake lines from the old master. They all came loose. Thank you answered prayers. I tried in vain to remove the booster and the MC as one unit. After trying all angles about six times just because I'm stubborn I relented and pulled the master off. Then wrestled out the vacuum booster. Got a little scary flexing the line to the tach but she gave way politely. All the brake fluid helped lubricate everything. Hahahahaha.

I swapped brackets on the boosters and made sure the new went together like the old then put in the car. Got it all bolted in and slapped on the MC. That's where I went wrong. Continuing I made three six inch pieces of brake line with a fitting on one end. The other end was cut line with a rubber hose that led to the res. I put those lines on the MC and connected the res hoses...then used the brake pedal to bleed the master...the hoses leading to the dummy lines I put on kept feeding from the res to the MC. Credit to this video: https://youtu.be/JndDMtZHGjc

Once the MC was bled I went to hook up the old brake lines. Rear was good. Driver was good. Passenger front fitting was flared from being over tightened and would not fit in the new MC. Bubble flared under the hood (fun) and put on a new fitting. Then I set to bleed the brakes. Rears were bleeding but fronts...NOTHING. Barely any movement. (pay attention folks)

I could also hear a "honking" coming from my carb every time I hit the brake. I figured this was the new vacuum pushing air into the manifold like a 2000cc tuba being played for the first time. Concerned that it was also correlating to the failure of the front to bleed I called Vicks, the part supplier. The guy who answered was about as helpful as a master mechanic leaning over the engine bay. I told him my procedure and observations. He said the honking is just the vacuum pushing air. It then dawned on my that the booster is pneumatic and the MC is hydraulic. Two separate systems..so of course that vacuum had nothing to do with bleeding the MC. BUT....

The guy from Vicks said he knew why I wasn't bleeding and he said, "this is going to annoy you but this is what's happening."

He explained that the push rod on the booster has an adjustment nut to set the depth of the push rod and where it connects the plunger of the MC. Screwed in too far and you lose needed depth of action in the MC. Screwed out too far and the MC stays constantly pushed in which could drag the brakes. He helped me set it..so I thought.

I removed the MC and adjusted the plunger. That required installing and removing the MC about 10 more times...not bolting it on...just slipping it on. I used a bit of paint on the end of the vacuum rod to tell when it was contacting the MC. Once I got it to where I thought he told me I put it all back together. Still no bleed.

I repeated the MC bleed again with the mock lines and rubber hoses to the res. Hoping to get air out of the MC. I noticed that flow through the rear brake line port on the MC was strong and had pressure (don't ask how I know...just realize eye rinses are a thing)....the front line ports were barely pushing fluid up an air filled line to the res. It was a head scratcher but I knew no better. I figured...pressure would come when the line was on. Yeah...I'm shaking my head at that one too....now. LOL

Reconnected everything....no bleed. Repeated above...no bleed. Swore a lot then tried again. No bleed. Took off the lines and noticed fluid flowing out of the rear MC port but not the front...I figured air was trapped. Had brother pump brakes while covering the rear and front ports. Could feel it in the pedal and my fingers...pressure in rear port...none in front. Head scratcher. Gave up and took a dinner break. It had "only" been 5 hours since I started. I figured by 8pm food might help. Mechanic's fatigue was definitely setting in.

During dinner I researched how a MC works. The physics of it. A video on the internal parts was very helpful. Then light dawned. The plungers in the MC were possibly not lining up with the port that fed the front lines of the MC with fluid. Eureka! I wolfed down dinner and raced out to the garage. I put the dummy hoses on the MC again and had my brother pump the pedal. The rear hose shot like a garden hose when I put my thumb on it. That was fun. Atomized brake fluid is yummy.

When I covered the front lines there was NOTHING. I removed the dummy lines and had him pump. I noticed it before but didn't think anything of it. A little fluid was getting pushed up and sucked back in the MC port hole for the front lines every time he worked the pedal...but that was it. It was just pushing fluid in and out...it wasn't pumping it from the res. I put a rubber hose on the MC feeder port where the res dumps in fluid and blew on it. NOTHING. Totally blocked. Eureka again! The plunger inside the MC was not properly aligned. I had set it to where Vicks told me but apparently misinterpreted the instructions. Now knowing what to look for I set out to find the setting by trial and error.

Had to remove the MC and adjust the push rod on the vacuum booster. It took about six tries at finding the setting where if I pushed in on the master (instead of bolting it down) and held it while my brother hit the brake pedal I would get good pressure in the front ports. Put it all back together and the second I connected the front feed from the res its started to overflow the front line ports of the MC. YES!!!! SUCCESS!!! But then realized i needed to connect those lines BEFORE the feeds from the res...or I was gonna lose all the res fluid before I could arduously connect the three brake lines.

As luck would have it the drivers line started giving us problems. Tried 2 bubble flares under the hood so we could install a new fitting and ran out of line. Sidetracked to replace the front driver brake line. Ten minutes later...continued.

Got everything together and holy crap it bled like a stuck pig! Did two rounds front and back. Plan on doing another round on Monday but OMG does this things have brakes!!!! It actually has better pedal feel than my 2010 Jeep Wrangler! Okay...maybe not quite but damn close for a 1979! Braking is so smooth and the pedal is hard right up high. I get hard stop right about where the brake pedal meets the darn gas pedal now!!!!

So the moral? When you install your MC on a new vacuum booster set your push rod from the booster by feel. The push rod should be adjusted to where if you hold the MC against the booster...about a 1/4" depress on the brake pedal starts to push the MC away from the booster housing. Bolt it on and hook up your res lines to test your push rod depth setting. If you are getting a solid flow out the brake line ports of your MC then your MC plungers/seals are aligned in a good place, your push rod setting is acceptable. Proceed with the install and bleed away!

All in all...if not for the INCREDIBLY steep learning curve...this is a pretty easy job.

Caio!

[blazingspider]

Glad to hear you got it all sorted out!! As you discovered the hard way, the plunger depth of the brake booster is critical to brake functionality.

One note of caution for you here!! Setting the plunger out too far will result in locking up the front brakes once everything gets up to temp. You'll know if this happens as the car will feel like there's been a loss of power when it's really the front brakes dragging when you're not depressing the brake pedal. The rotors will get very hot as well.

FWIW, I never felt the need to bench bleed the mc or bleed it in place. The end result is usually geting brake fluid all over the place and we all know what brake fluid does to paint!

As far as beelding the system goes, I've found that using a power bleeder is my preferred method. I don't fill the container with brake fluid though, I just pump it up with air, attach it to the reservoir, crack the furthest brake bleeder and watch the flow.

Happy Thanksgiving!!

Blaze

[moss1972]

Glad to hear you got it all sorted out!! As you discovered the hard way, the plunger depth of the brake booster is critical to brake functionality.

One note of caution for you here!! Setting the plunger out too far will result in locking up the front brakes once everything gets up to temp. You'll know if this happens as the car will feel like there's been a loss of power when it's really the front brakes dragging when you're not depressing the brake pedal. The rotors will get very hot as well.

FWIW, I never felt the need to bench bleed the mc or bleed it in place. The end result is usually geting brake fluid all over the place and we all know what brake fluid does to paint!

As far as beelding the system goes, I've found that using a power bleeder is my preferred method. I don't fill the container with brake fluid though, I just pump it up with air, attach it to the reservoir, crack the furthest brake bleeder and watch the flow.

Happy Thanksgiving!!

Blaze

Yes...Vicks warned me of that. Currently after the drive home Tues night all seems well. I will monitor it and know how to adjust it now. Learning the hard way is one way that actually helps to teach every last detail. Sometimes it's what I need. Though I prefer easy ways...they often come with experience. I appreciate you sharing yours.

I'm going to vacuum bleed now that everything is flowing. And you are correct...if the MC and plunger is setup correctly the master floods readily with fluid by gravity alone.

Happy Thanksgiving to you, my friend.

[18Fiatsandcounting]

Wow, Steve, that's quite the epic tome that you've written there! All you need is a damsel in distress, preferably Italian, and you'd be ready for release as a Kindle edition.

I haven't ever sat down and really tried to figure out the master cylinder, as it's always seemed to work fine when I've replaced it or rebuilt it. But, I have gotten the sense (such as you've alluded to) that the internals of the MC have to be lined up so that brake fluid can be pulled from the reservoir and then pressurized to the 4 calipers when you press on the brakes. Presumably, the MC is "refilled" from the reservoir when you release the brake pedal and things return to their rest position, but that's just a presumption. Presumably, that's a presumption. Hmmm... I haven't even started to hit the eggnog yet...

One day I'll have to study all this, as I've got a few old master cylinders stashed away. So now you've given me something else to do, durn your hide!

But seriously, glad you got it straightened out and thanks for the info. When the braking system is working properly, the spiders have pretty good brakes what with 4-wheel discs, a relatively light car, low to the ground, close to 50/50 weight distribution front to rear, etc. I know they can't compete with modern cars, but still pretty good for that era.

-Bryan

[moss1972]

Wow, Steve, that's quite the epic tome that you've written there! All you need is a damsel in distress, preferably Italian, and you'd be ready for release as a Kindle edition.

I haven't ever sat down and really tried to figure out the master cylinder, as it's always seemed to work fine when I've replaced it or rebuilt it. But, I have gotten the sense (such as you've alluded to) that the internals of the MC have to be lined up so that brake fluid can be pulled from the reservoir and then pressurized to the 4 calipers when you press on the brakes. Presumably, the MC is "refilled" from the reservoir when you release the brake pedal and things return to their rest position, but that's just a presumption. Presumably, that's a presumption. Hmmm... I haven't even started to hit the eggnog yet...

One day I'll have to study all this, as I've got a few old master cylinders stashed away. So now you've given me something else to do, durn your hide!

But seriously, glad you got it straightened out and thanks for the info. When the braking system is working properly, the spiders have pretty good brakes what with 4-wheel discs, a relatively light car, low to the ground, close to 50/50 weight distribution front to rear, etc. I know they can't compete with modern cars, but still pretty good for that era.

-Bryan

I won't lie. I did hit the eggnog before I wrote that. LOL Your presumption is correct. It's really cool. Check it out some time.

https://www.youtube.com/watch?v=Xk2zGvKfrhc

This is fantastic: https://www.youtube.com/watch?v=I4Ka4__ws1E

Funny mention the comp to modern cars. My brother bought a 2019 Spider. So fun to drive. REALLY fun. In fact I started thinking the other night when I took it home...trade? Then I ready they stopped making them...but the Mazda Miata MX-5 is virtually the same exact car. Like literally...same interior and sourced parts. Its within my reach financially....but you can't work on the darn thing. And they are so full of pesky electronics. Then I realized my 1979 is ONE of a kind. I've got it so dialed in, and am getting to know it in and out. Seems foolish to get rid of something I would NEVER be able to find again for some other car that is so commonplace.

Anyway...caio for now!

[18Fiatsandcounting]

https://www.youtube.com/watch?v=Xk2zGvKfrhc

This is fantastic: https://www.youtube.com/watch?v=I4Ka4__ws1E

Thanks for those links.

On the second one, I had to briefly look away when he started wiping the master cylinder with an old pair of underwear. Dude, get some shop rags! And maybe put a pair of circlip pliers on your Christmas wish list as well...

Also, see how easy it is to remove a brake booster and master cylinder? I don't know why all these folks keep complaining about how hard it is in a Fiat spider. I mean, look at all the room the guy in the video had to work with, and how easily all those rusted nuts and fittings came loose!

Also, fine by me if you post Fiat repair information while hitting the eggnog.

-Bryan