Where is my brake pressure??

[Erkenbrand]

This thing is driving me crazy. I'm working on my 77, slowly bringing it back after I bought it from a guy who had it sitting in his garage for a decade.

All of the calipers were seized, and the master cylinder wouldn't hold fluid. So, I've replaced:
- All 4 calipers with new.
- The compensator in the rear with new.
- The master cylinder with new (twice now).
- And all of the pads, rotors, and flex hoses, including the flex hose from the rear axle to body.

I made sure the castle nut off the booster was 1MM proud of the face. I've now adjusted it out to 1.5MM proud.

I bled the master cylinder, and have bled the full system. Repeatedly. I can not get any pedal. I thought I had a bad new master cylinder, so ordered another from another supplier, but still having the same problem. This isn't my first rodeo - I've replaced many master cylinders, and entire systems on all sorts of vehicles. But this one has me stumped. I've tried reverse bleeding, gravity bleeding, two-man, etc. I get fluid with no air to all 4 corners. Everything looks great. But still no pedal.

When bleeding the rears I keep the axle supported to give load to the compensator.

The most I get is a bit of brake drag at the very bottom of the peal stroke. But not other pressure.

Any tips? Clues? Soon, I'm going to cough up the money to send it to a mechanic. And I very much would rather not.

Thanks for any help!

[18Fiatsandcounting]

It sounds like you are doing everything right, but this does seem to happen from time to time, and I'm not sure that anyone has definitively figured out what's going on. Many times the issue is that the master cylinder is not sucking enough fluid from the reservoir as you bleed the brakes, so the master cylinder still has air in it and doesn't develop much pedal feel. If you had air in the lines or the wheel calipers, you would get sponginess but at least some pedal feel. Since you get no pedal at all, it sounds to me like the MC is at fault.

A question: When you bleed the brakes, is the reservoir level going down pretty quickly? For a properly working system, it doesn't take many pumps of the pedal (perhaps ten) on a single open caliper before the reservoir is empty. If your reservoir is dropping much more slowly, then it appears that fluid isn't being drawn into the MC as much as it should.

I know this is a pain, but if it were me, I would loosen the 3 high pressure lines from the MC (2 front and 1 rear) and use an eyedropper to add fluid to these openings in the MC. Perhaps tap gently on the MC with a hammer as you do this. Sometimes letting it sit with fluid in the connection cavity helps, perhaps an hour or so. Then another dose with the eyedropper, etc.

Usually, fluid eventually gets around inside the MC and then all works well, but it can be frustrating until it does, and it kinda sounds like that's where you're at.

-Bryan

[Erkenbrand]

That's an interesting point. To answer your question - I can empty the reservoir really quickly when bleeding the fronts. But when bleeding the rears it wasn't moving anywhere nearly as much fluid. It hadn't occurred to me before.

I did change out the lines from the reservoir to the MC since the old ones were a mess. Maybe it's time for round 100 pulling the pressure lines off and trying again.

I really appreciate the pointer. I'm trying to convince myself to keep pressing on. Part of the frustration - I've replaced every suspension component (using KYB shocks / AR lower springs, etc), every steering component, carb (Weber 32/36), and updated the ignition with a Pertronix kit. I'd really like to drive it now.

I'll give this a try and report back!

[spider2081]

I think there is no internal mechanical connection for the master cylinder front and rear pistons. There has to be some pressure from the front brakes system for the rear brake master cylinder piston to move. I bleed the right front caliper first when the master cylinder is replaced. That usually gets a little peddle then I go to the right rear.

[18Fiatsandcounting]

It kinda sounds like your fronts may be OK but the issue is with the rears? What I have done in the past is to put a clear plastic hose over the rear caliper bleed nipples, run that into a mason jar or the like with an airtight seal through the lid, and then use another clear plastic tube in that same mason jar lid to suck the fluid through the lines. Sort of a poor man's fluid trap, as not even poor men like to get a mouthful of brake fluid... I also have a lab grade vacuum pump which I have used, but I don't expect most people to have this sitting around in their garage.

Anyway, the idea is to get fluid moving really fast through the rear lines, and that often helps dislodge bubbles and the like. Make sure you keep enough fluid in the reservoir, as if something does "break loose" (no pun intended), you could deplete the reservoir pretty quick and then have to start over.

-Bryan

[18Fiatsandcounting]

I think there is no internal mechanical connection for the master cylinder front and rear pistons. There has to be some pressure from the front brakes system for the rear brake master cylinder piston to move. I bleed the right front caliper first when the master cylinder is replaced. That usually gets a little peddle then I go to the right rear.

That's a good point Spider2081, and I hadn't thought of it. I usually do two complete passes around the car, bleeding each caliper twice, so that would be 8 bleeds, and maybe that does something similar. Stock up on brake fluid!

-Bryan

[DieselSpider]

I used a vacuum bleeder and it got er done on the first shot. With the rear axle on jack stands it was right rear, left rear, right front, left front and all was good. Sometimes you do have to bleed the master cylinder back into the reservoir first to get all the air out but I didn't need to. I believe I used a Beck and Arnley master cylinder from the local auto parts store.

[18Fiatsandcounting]

There has to be some pressure from the front brakes system for the rear brake master cylinder piston to move.

I thought about this some more, and it may not be quite right. I seem to recall that if the front brakes failed (loss of pressure), as you pushed the brake pedal almost down to the floor, the front end of the MC piston for the front brakes would directly push on the rear end of the MC piston for the rear circuit. So, you would get some braking action, admittedly very weak.

But, my poor memory could be deceiving me, so if anyone knows differently, please correct me.

-Bryan

[Erkenbrand]

Follow up: This weekend I was able to get into the garage and do some more. Mixed results.

I pulled the reservoir hoses from another 77 I have since they are known good, and of proper length. I then bled the master cylinder again, and spent an hour bleeding, tapping on it with a ball peen, etc. There was no air for a long time.

Then, I went around the car re-bleeding everything.

I still can't get good fluid flow to the rears. I can get a small amount, and get the air out, but it's not the right quantity. I have the car jacked up from the pumpkin. I'm on the verge of bypassing the compensator and putting in a splitter just to see if that makes a difference.

The fronts bled quickly.

At the end, I did have front brakes for a bit. I even took it on the road. After a couple of miles, I had no pedal again except just a bit at the very end of the pedal stroke.

At least I had the hand brake nicely tuned.

At this point, I am more than happy to take this to a garage if I knew somebody who could tackle Fiats. After all of the other changes I've made, it needs an alignment anyway. And it needs to pass New York State inspection. I just don't know what to try anymore . . . and I'm out of patience.

[spider2081]

I still can't get good fluid flow to the rears.

I think the lowest point in the Spider brake system is the "T" mounted to the rear axle. I have seen a few of them clogged with corrosion. Have you checked for fluid flow from the hose to the "T"? If the fluid flows freely from gravity the "T" may be one of the issues.
Jacking the car from the differential should open the compensator However if it is not adjusted properly it may not be opening. Some folks prefer to disconnect the short rod at the differential bracket and push it up as far as it will go. This is a more positive way to be sure the compensator is open.

[Erkenbrand]

Thanks for the advice . . . I was getting good flow from the down the hose to the axle. I replaced that hose, and had plenty of fluid pour out while I had it apart.

I replaced the compensator, but I'll give unhooking and pushing the arm up a try. At this point, I'm looking for anything.

[Mheinle]

Hey There,
Curious if you found a solution to this issue? I am having the same issue, I have taken the car to a shop for a solid flush and they cannot figure out what the heck is wrong. They have confirmed the air is out of the system but the pedal travels to the floor before the brakes drag at all. Any follow-up would be appreciated.

[Mheinle]

Hello again,
Replying back as I went and picked the car up last night and babied it home with little to brake with. I decided to just start at the basics knowing there was no air in the system and I found my trouble. It ended up being two things.
1. The vacuum hose was not tightened down well, I am not sure if I over looked it or if it just worked itself free but the hose clamp was super loose. I replaced it with a new clamp and got it good and tight on both ends, a test ride revealed better brakes, not perfect but better.
2. I separated the master cylinder from the booster and increased the length of the booster push rod. This made a huge difference, I can now stopp the car with confidence. I think they could be better through further adjustment of the push rod.

I just wanted to share my findings incase it helps others.