Brake Booster Replacement - a Hint

[geoff]

yes Bryan, my logic is that as the brake pipe makes an upward loop and then down into the M/C, it is impossible not to have air in that part when reconnecting the pipe - so that would mean if doing the bleeding the 'normal' way would result in having to push that air right through the system until it comes out of the rear nipple. The way I did it, I reconnected the pipe to the M/C as the fluid was dripping from the pipe.
I had topped off the O/L from the M/C first so there was not going to be any air in that part of the line as I reconnected it.

[DieselSpider]

yes Bryan, my logic is that as the brake pipe makes an upward loop and then down into the M/C, it is impossible not to have air in that part when reconnecting the pipe - so that would mean if doing the bleeding the 'normal' way would result in having to push that air right through the system until it comes out of the rear nipple. The way I did it, I reconnected the pipe to the M/C as the fluid was dripping from the pipe.
I had topped off the O/L from the M/C first so there was not going to be any air in that part of the line as I reconnected it.

Yes that bubble would require that new, fresh, clean brake fluid be pumped through the entire system resulting in more potential contaminants being purged from it ensuring the longest possible life from all components. I will pump out the entire volume of the system twice to better ensure a clean well purged system after replacing a hydraulic brake component.

[geoff]

well it has been a while since my last post on this. -
Still things are not right!
After a short road test, all the brakes locked up. Yes, I decided that the servo (booster) rod which activates the master cylinder was out of adjustment, so I reduced its length as much as it was possible by adjusting the nut and tried once more.
Still the same result ! - so this time in order to shorten it still further i am going to have to remove and discard the locknut.
I cannot find in my manuals any guidance on the correct adjustment. Does anyone know ?
regards to all
geoff

[aj81spider]

According to the Fiat shop manual (page 33-18) the plunger should protrude between 0.0408 and 0.0758 inches (1.035 to 1.925 mm).

[18Fiatsandcounting]

Geoff, let us know if the adjustment helps. If not, try disconnecting the vacuum hose to the brake booster and then driving the car. You'll need to plug up the hose that goes to the intake manifold so the engine will run of course.

Here's my thinking: If the brakes still lock up, then it's likely your plunger is not adjusted correctly, as already stated. If the brakes don't lock up, then it would seem that the booster is somehow activating the brakes on its own, perhaps because of a faulty unit, or a weak internal spring, the pedal linkage adjustment is off, or ???

You could also try this: Run the engine with the car in neutral, and carefully observe the brake pedal close up. Does it move at all by itself when you rev the engine, let it spin down again, wide open throttle then release, etc? If the pedal moves, it means your booster is acting on its own and applying the brakes.

It's something I've always wondered, namely, could an incorrect or misadjusted booster start to apply the brakes (without the driver pressing the pedal) if given sufficient vacuum?

-Bryan

[Nut124]

My experience would suggest that if Fiat front brakes lock up, the tires are junk.

If rear brakes lock, then the rear regulator could be the problem, or tires.

[geoff]

thanks Bryan for that useful advice. Here is what just happened:
Believing that the actuating pin in the servo was out too far, I reduced its effective length and once again a test run.
Once again all brakes locked after a few presses on the brake - coupled with no brake pedal travel.
I was beginning to think that the servo was faulty. Got the car home by slackening off the pipes at the m/c and allowing some fluid to be released.
Today, after doing nothing else - I retightened the pipe connections and went for a short drive. All seems much better now. (still a little more bleeding to do to get it perfect, but otherwise normal).

I cannot fathom out why the situation has corrected itself after merely allowing a little fluid to be released from the M/C? It is as though the system was under pressure even without the brake pedal being pressed.

Any ideas ?

[18Fiatsandcounting]

Any ideas ?

I have to admit I'm baffled. If the brakes start out fine, but then they start dragging or locking up as they are used, that usually is a sign of one or more problems: 1) collapsed brake hose acting as a "one way valve", 2) caliper pistons not retracting as they should when the brake pedal is released.

But it sounds like something is causing hydraulic pressure to be maintained in your brake lines even when the pedal isn't pressed. So, as you use the brakes, the pressure keeps building up until the brakes drag or lock up.

A couple questions:
1) When you released fluid from the MC, did it spurt out, or just kinda ooze out? In other words, could you tell if it was under pressure?
2) This may be hard to do since the problem appears fixed, but would the brakes slowly lock up if you pumped them while the car was sitting with the engine off?

What could be happening is that your MC is very good at pumping up the pressure in the lines, but for some reason, it's not releasing the pressure when the brake pedal is released. Perhaps a bad MC, or not quite the right one for your car?

-Bryan

[spider2081]

According to the Fiat shop manual (page 33-18) the plunger should protrude between 0.0408 and 0.0758 inches (1.035 to 1.925 mm).

I'm not sure this applies to all replacement booster, master cylinder combinations.
I think measuring the original length of the rod protruding from the face of the original booster body is a critical length when replacing the booster. Adjust the rod for the original protrusion on the new booster, if keeping the original master cylinder.
I also think measuring the depth of the original master cylinder cup is critical when replacing the master cylinder. If the 2 master cylinders have the same cup depth the rod length should be ok. If the cup depths are different the rod length needs to be adjusted to compensate.

[courtenay]

I had to toss the lock nut on the rod when I did mine. It protruded too far with the lock nut on.

[SteinOnkel]

I had to toss the lock nut on the rod when I did mine. It protruded too far with the lock nut on.

Same. Replaced the mc and booster at the same time.

Idiotic design, never had to deal with this nonsense on any other car. It's always the pedal that's adjusted (and sometimes not even that) and not the plunger in the servo. Also, measuring the protrusion is an exercise in patience.

[courtenay]

I had to toss the lock nut on the rod when I did mine. It protruded too far with the lock nut on.

Same. Replaced the mc and booster at the same time.

Idiotic design, never had to deal with this nonsense on any other car. It's always the pedal that's adjusted (and sometimes not even that) and not the plunger in the servo. Also, measuring the protrusion is an exercise in patience.

Yup- I found that the thickness of two business cards was about right.

[geoff]

well I checked on my old servo and find that the rod is not protruding at all - in fact it is 5mm. inside the servo housing.
I think my new servo is working ok now - I have removed the locknut and screwed in the rod as far as is possible and I think I just got away with it. I think it is ok now !!

[RRoller123]

I found the exact same thing regarding my new booster, a couple years back now. I had to adjust it as far IN as it would go to get at the correct protrusion. But I did NOT have to remove the nut on the end. It barely made the spec though, screwed all the way in. But since it did actually make the spec, no harm, no foul.

Pete