Not your typical locked brake problem

So my brakes were intermittently locking. Not an uncommon problem at all, we all know how to fix this. I was due for a fresh brake pads anyway. So I began with that. All of the pistons on all of the wheels were properly extended. When I pushed them back in, all went in smoothly. It doesn’t seem to be a caliper problem.

Since I was there, I also replaced all three brake hoses. Also replaced a questionable brake bottle. Pumped a quart and a half of fluid through the system. Brakes still locked.

So I took off the reaction valve. I cleaned it up, made sure its action was smooth and that the sealing surfaces actually sealed. Then I applied some silicone grease to the small piston in the master. This master cylinder has maybe 400 miles on it. When I put the piston back into place, I inserted the diaphragm gadget and had my helper press the brake. the piston extended with authority and snapped right back into place when the brake was released. Reassembled. Brakes still lock. IR thermometer says front brakes are at 400 degrees.

So it still must be vacuum, right? I plumbed a vacuum gauge into the rear port of the booster. Brakes locked, vacuum at the rear capsule steady @ 20 lbs. It’s absolutely not vacuum related.

Now I’m stuck. I have a relatively new slave & master. I have new hoses. Fluid changed. Calipers work smoothly. Vacuum system sound, reaction valve sealing. Brakes lock. Anyone?

Do you know whether it’s the front AND rears that are locking or just the fronts?

Do they stay locked? Or release slowly?

How long has this been going on??

There is a brake pedal adjustment on the back of the pedal box at the firewall. If this is tightened up too much, you will not be letting the master cylinder piston retract enough that fluid can return from the brake circuit (under pressure) back into the master cylinder reservoir through a small hole.

This small hole is normally “open” and connects the master cylinder to the fluid reservoir. When the brake pedal is pushed, the seal on the piston wipes past the hole thus locking all the fluid in the mastercylinder. Further pedal movement pushes fluid out of the master cylinder into the brake circuit (the booster)

Dennis 69 OTS

I kinda have this same problem, being worked right now, one test I did where I knelt down and operated the brake pedal by hand. When I operated it I heard a gurgling sound and when I released it the piston held its position for a second before it snapped back to full out. This is in spite of my bleeding the complete system, twice.

My master and booster are also new with maybe 150 miles on them. With your problem being almost identical to mine I’m wondering if the return spring in the master isn’t as strong as the original, hence the problem of slow return?

With your new Master did you adjust the screw I mentioned to take up the free play in the brake pedal and no more?
Dennis 60 OTS

No, and I still have the problem that I thought I’d fixed with all new vacuum hoses. There’s plenty of free play in the pedal to master cylinder coupling so the pedal isn’t causing the problem you related, not in my case. But the cylinder doesn’t come back quickly. That’s easy to detect if you operate the pedal by hand and listen. This is all with the engine off.

Les, if you think the spring is weak and the seals are new they might have swollen and now resist the spring - we had an episode with my re-sealed master cylinder and the piston would stick in the bore a little and only sometimes. New seals fixed it - maybe some oil got on them, enough to make them swell. It felt as if the spring was weak (but strong enough to hold the brakes?) and it would not slide out easily. Friction.
I have to mention they wouldn’t really bleed either but this might be unrelated. Now they’re great.
In any case with a recent master cylinder replacement that would be a good thing to look at. I think the slightest amount of oil can, after a while, do that. Pain to check… I know it is not as accessible.

That is a strange phenomenon. The spring that pulls the master cylinder piston back out is a strong one and therefore tough to install so I suspect that some clearance is too tight and not allowing the piston to snap back easily as it should. There is a plastic bushing the piston rides in. Aftermarket versions sometimes have a metal bushing. If you have an aftermarket version, maybe the rear seal is just plain too tight on the piston or the front seal is too tight in the bore

It might be that your piston is not retracting enough (as you say) and the piston seal is therefore not uncovering the little hole that allows fluid to go back to the master cylinder.

Let me make a suggestion!!

Put a string on your brake pedal. Take the car for a drive and when you feel the brakes stick, pull the string to be sure the brake piston is all the way back. If the brake sticking goes away when you do this, the problem will be fixed when the stiff piston retracting problem is fixed

Dennis 69 OTS

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I have free play in the pedal, so I don’t think the adjustment is off. If anything, it’s too loose. Once it makes contact, it’s firm. I had replaced the master, the servo and the slave as part of a big rehab about four years ago. Since then the car has mostly shuttled around between storage, my shop, and Lippincott’s for some paint work. Brakes worked fine until the car came out of storage this spring, so it’s something that worked and then didn’t . The front brake bottle had half emptied itself over the winter. I couldn’t tell if it was the line or the bottle, so I replaced both. I suppose it’s possible that I left some air in the system somewhere, my next step is to review all my work, starting with bleeding. But the problem began before I even started work, so I don’t have high hopes. My next step would be rebuilding a cylinder…but which one? The master or slave?

The problem seems to happen spontaneously, just to make it more fun. At first, I thought the engine was cutting out. But it was only struggling to overcome the brake. Since vacuum doesn’t fluctuate, it pretty much has to be hydraulic. Temps at the front rotors are 400F, rears 250F. So I think I only have a problem with the fronts.

Les: the gurgling sound may be the sound of grease fluttering in the reaction valve. I found that the reaction valve was coated with grease from the factory. Have someone pump the pedal while you listen.

If the front bottle emptied and you do not know where the fluid went, it went into the master cylinder, and from there into the servo rear section and from there it leaked past the rear servo seal into the rubber diaphragm area of the servo. But this is only correct if the front bottle feeds the Brake Master cylinder. If so, then your servo needs rebuilding.

If your front bottle’s fluid is directly connected to the servo, then the fluid must have leaked out somewhere else.

Dennis 69 OTS

Hi Michael, I had a similar thing happen, the brakes locking after I had driven for some time. Once, I was stuck and just sat there on the side of the road for a half an hour and strangely they unlocked themselves(new master and servo). So I headed home they started to lock up again! Made it into the garage and examined everything…my new vacuum hose was collapsed. Usual vender BS. I ordered some hose from another vendor, problem solved.

This is the brake bottle that feeds the front of the slave cylinder. It didn’t leak out the back. It dripped down to the aluminum underpanel from the hose connection on the bottle. The bottle itself was in poor shape. Replacing the bottle and hose appears to have fixed that problem.

It originally seemed like a vacuum problem, but it’s absolutely not. I have a vacuum gauge tapped in between the reaction valve and the rear of the servo, so I know when the servo is pushing. When the brakes lock, I still have 20 inches. After the car has cooled for 8 hours, I still have 13 inches of vacuum and it holds there.

I should add that in ordinary driving, vacuum is around 20 inches. When I press on the pedal, vacuum level in the rear of the servo drops progressively with pedal pressure. It only reaches zero if I stand on it. I don’t know if this is normal, but it seems right.

I struggled with the same or similar problem a few years back. I Suspected heat to be a factor and used a cooling spray on various parts to isolate the cause.
I found that the end of the MS where the reaction valve is mounted reacted to cooling.
Not having the drawings handy on my phone, I’m just going to call it the small plate that pushes the small piston that operates the reaction valve. It had sharp edges from the manufacturing process that I suspect caused it to stick in the activated position of it had been turned around from its original position.
I filed the edges off and have not had sticking brakes since.
I posted something to that effect here and on the UK forum as well I believe.
Cheers … Ole

Thanks, Ole. I’m going to have to take it for a run and keep a much closer eye on vacuum. I may have missed something subtle.If that plate sticks, I wonder if the main piston can return to it’s fully retracted position. That may trap fluid pressure in the line.

Hi Guys
On my S3 I had a similar problem with brakes sticking on. I tried the usual fixes until the final thing I noticed was that the piston in the mc was slow to return,
and probably not completely, causing all brakes to hang on.
I decided to fit new seals in the MC which was only a couple of years old. When I did so the large nylon bush with the O ring on it, part 10878 in the S 3 parts diagram was still very tight, preventing smooth operation and return of the piston. I swapped it out for an older bush that I had and lo and behold the pedal was as smooth as silk. I checked out the flapper part 10877 as mentioned by Ole, but this seemed smooth. I also used loads of red rubber grease in the reassembly, its compatible with brake fluid.
The MC is made by Caparo, and I have seen other complaints about the tight fit.
I think this is what poster Dennismo mentions in the thread.
Did this 8 months ago and so far not a sign of earlier problems.
Well worth investigation

Michael, based on all your observations, I would guess at the servo/booster unit. But that would be a guess. However, being cheap, I like to be pretty sure which component is faulty before I spend money. And being lazy I like to know which component is faulty before removing it. I would suggest the following:

  1. Your vacuum tests do seem to rule that out. But to be sure, I would simply disconnect the vacuum at the intake manifold and plug the manifold port. Pump the brakes to empty the system/reservoir of vacuum. Now the vacuum cannot be the issue. See if the brakes now get tight. I always warn when doing this, you have disabled the brake’s backup system, so drive carefully.
  2. Now, get the brakes to get tight. Then, first crack open the brake line from the M/C to the servo at the servo. If the brakes release, the problem is in the M/C, pedal adjustment, or the metal line from the M/C to the servo is blocked.
  3. If step 2 did not release brakes, now crack open the line from the front brakes to the servo, at the servo. If the brakes now release, the issue is the servo.
  4. If step 3 did not release the brakes, now crack open the line at one of the front calipers. If the brake releases, the issue is line/hose between the caliper and the servo. If that caliper is still tight, the caliper is the issue.
    Good luck,
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So it would seem that either the MC or the servo/slave combo I installed four years ago is the problem. Maybe I’ve had this problem since they were installed, and it’s just too intermittent to have noticed. Maybe getting it’s worse, and someday the car will stop moving forever. At that point, it will be easy to diagnose. I think my next step is to disassemble the MC and see what’s going on there. Pulling the servo is the bigger/more expensive problem, so it can’t be that, right?

Michael, you know my opinion, It can be the servo. And for the amount of time it takes to troubleshoot, even if that troubleshooting does not help, I would still do it.

Let’s review the MC being the problem.

IF your vacuum readings are correct, THEN it should not be the small piston or the small metal plate. Disconnecting the vacuum source would confirm.

IF the fronts are locking and the rears are not, THEN it should not be the M/C, as that would apply the brakes to each evenly. Although at 250 F on the rears, they may be dragging too.

IF the M/C piston is not fully returning, THEN I would think you would have excessive free play in the brake pedal that you could easily observe.

None of this confirms to me that the M/C is not the problem, as one can misinterpret results. I am just going by what you have reported.

I’m just postponing the problem. I’m a bit disgusted, because the MC, servo and slave are relatively fresh and have very few miles on them. These were new parts, not rebuilds, which leaves me with doubts about the quality of the parts supply chain. I think the total cost for the parts at the time was around $1000, and all the work of R&R. Now it has to be done again, and I’m rebuilding, not simply replacing.

Disconnecting the vacuum won’t do, because the problem only happens after some drive time, and even then it’s intermittent. Whether it’s heat or random positioning, I don’t know. I’m hesitant to drive the car without vacuum in order to test whether the master is dicey. Anyway, the only way I’ll be happy is if I see the insides of each part. So wish me luck, MC goes first.