Brake Booster or ?

Stophos · Stophos's Garage

I have a problem with my 88 930 that I thought I had narrowed down to my brake booster but I am having second thoughts. The problem is that after even a very short ride all four brakes do not fully release. Initially I thought master cylinder and replaced it. No difference. Then the hoses, then rebuilt the calipers. I rebuilt the peddle set thinking it was sticking. No help. I took it for a ride pulled it on the lift and the brakes were locked up so I disconnected the booster rod from the peddle set thinking this would eliminate a mis-adjusted rod. No difference. I removed the booster assuming it to be the problem but I am thinking I am missing something. I removed the master cylinder from the booster and could hear pressure releasing which I think is normal. Visually it looks good and the piston moves freely. Before i purchase a booster can Anyone tell me what I may be missing?

Thanks - Chris

cgarr · cgarr's Garage

And there were no problems bleeding all 4? If you crack the line at the caliper will it release?

Are the pads correct? some can be tight on the caliper/to big for the holder?

Stophos · Stophos's Garage

No problems bleeding. I do not recall for sure but I pretty sure that if you crack the line it would release the pressure. Pads are correct 100%.

cgarr · cgarr's Garage

Well then its not in the caliper or pads. Sounds like the MC is not venting I would guess a miss adjusted rod but you said it was removed without helping?

I think its in the rod, loosen up the booster (4 nuts) when its locked up to release the rod and move it back slightly and see if it releases

Stophos · Stophos's Garage

Before removing the rod I first loosened the four base nuts on the booster which changed nothing so I then removed the rod. I assume with the rod removed the piston would be totally released and therefore the master cylinder would vent. Is it possible that the master vents before it is fully released and the rod was too far out?

Walt Fricke · 09-20-2011, 08:52 PM

When you let off the brake, the springs inside the MC push the tandem pistons back. As the pistons move back they uncover a hole (one per piston) in the top of the MC. This hole leads to the reservoir. So as a result the fluid not only can move back into the MC from the lines to the calipers, but can also move up, if need be due to expansion, into the reservoir, or its connecting tubes.

If the connecting rod is too long, the pistons can't retract far enough to uncover these holes.

Since completely removing the rods pretty much guarantees that these holes are uncovered, what might be next?

The traditional issue was the rubber brake lines. With age, some rubber inside can become detatched and form a flap, like a one way valve, hindering the ability of the fluid inside the calipers from moving back toward the MC. But it seems improbable that this would affect all four calipers.

How about you try this: go make it lock up, and then open the cap on the reservoir. If, somehow, the reservoir vent is plugged up, this could account for your problem. The fluid trying to back up into the reservoir due to heat expansion pressurizes the reservoir.

Maybe. But you will know instantly if this is the problem.

Stophos · Stophos's Garage

Craig and Walt, Thanks for your insight.

Walt, Being that the booster is out in order to do what you suggest I would have to put it back in the car and bleed the brakes. Rather than do this I would guess that I can simply blow back into the reservoir through one hose with the other hose and the clutch port sealed and see if it vents. Would you agree?

T77911S · 09-21-2011, 10:49 AM

walt, very nice write up.

since you have covered everything, what about the pads sticking, or the pistons sticking.
isnt the orientation of the piston important too?
just a thought.

Walt Fricke · 09-21-2011, 06:24 PM

Stophos

You could try that. It should tell you if the reservoir vent has somehow become plugged. It won't quite test the whole system from inside the MC to the outside air.

It might be simpler - just take the reservoir off and check that everything which should be open is.

Or, if the MC is in place, blow (fairly gently - 10 to 20 psi should be enough) through an opened bleed valve on a caliper. Fluid (if the system hasn't been drained) should emerge from wherever this model has its overflow from the reservoir. In the SCs it is in front of the left front wheel.

I can't imagine that at least one pad on each caliper is sticking. Of course, I am taking his word that this is the case. Same with pistons sticking. But on rereading the post, I see that he has replaced the rubber hoses, and rebuilt all calipers (or at least checked their action), which makes the flap theory and the sticking piston theory moot. Plus he had the car, in its brakes stuck on condition, up on a lift and confirmed that all four wheels wouldn't rotate (or at least did so reluctantly).

The piston orientation on the single piston per side calipers is important, but only to pad wear. The cut-out part is intended to have the piston exert less pressure on the leading edge of the pad. Since this is where the friction/heat is greatest, reducing pressure there helps keep the pad wear even. Otherwise you get taper wear, and find that the leading edge is rather thinner than the trailing edge. Not sure this hurts braking, but it hurts the pocket book as you have to replace pads sooner. Besides, this is a 930 we are talking about, and it has 4 piston calipers. The leading edge piston is smaller in diameter than the trailing edge one to produce this differential pad pressure.

Mind you, I have no experience with the vent being blocked, and we have to assume that your new MC is good. Removing the actuating rod between the booster and the MC (if that is what was done) pretty much guarantees that the booster (with no relief of the unwanted residual pressure) is unlikely to have anything to do with this problem.

sjsst41 · 10-01-2011, 07:20 AM

Chris - any update on the diagnosis/issue with the dragging brakes?

I have EXACTLY the same issue on my wifes '83 911 SC that I am trying to resolve.

Haden't had the car on the road for two years, two calipers were seized. Replaced all 4 calipers. Drive the car for 10 min and all 4 wheels start to lockup/drag (there is residual pressure in all brake lines - I crack open the bleeder and I get a squirt of fluid which released pressure and the wheel turns with no issue)

Or - I let it sit for 15 minutes after a drive and they release (cool down).

replaced the master clylinder last night thinking that has to be the issue - with exactly the same response.

You can tell the brake pedal has less "play" after drving - due to the buildup of pressure.

Any further input is greatly appreciated !

Thanks

Steve

Stophos · Stophos's Garage

I finally got back to this and decided to re-installed the booster and master. I bled the brakes and went for a 10 minute stop and go ride. Came home and sure enough the brakes were once again locking up. I removed the brake rod and no difference. I released the booster pressure by loosening the master cylinder and the brakes released. Tightened it all back up and went for a 15 minute ride and the brakes didn't lock up. Makes no sense to me. I haven't found time to take it out again but I suspect when I do it will do it again. I will post after the next ride.

sjsst41 · 10-01-2011, 12:11 PM

Have you tried to drive the car with the booster disconnected from the vacuum source? That was my next test.

Stophos · Stophos's Garage

I have not tried that.

sjsst41 · 05:18 PM

Just returned from a run - no difference.

There has to be some info posted on a resolution (someplace) - I can't imagine we're the only two who have experienced this type of brake issue.

I'll do some more searching and post up what I find.

Quote:

Originally Posted by Stophos

I have not tried that.

sjsst41 · 10-03-2011, 06:50 AM

I'm stumped - I did find threads that had folks with similar issues (residual brake pressure buildup) but none seemed to post a specific solution.

I plan to re-adjust the brake linkage this evening - to see if that has an impact.

Walt Fricke · 10-04-2011, 09:01 PM

Steve - you don't say that you have replaced the rubber brake lines. While this has not happened to me, it is pretty standard for someone who takes his car on the track or autocrosses it to replace the rubber ones with flourocarbon steel braided lines. Or with new rubber lines.

The common wisdom is that as the rubber degrades with time it is possible for a flap to form, and the flap may form in such a way as to create a one way valve - the one way being fluid to the caliper, thus blocking the return flow when the master cylinder returns to its "off" position, thus opening up the circuit to atmospheric pressure and allowing any pressure to bleed back to the MC, and through it if needed to the reservoir.

Sophos did this replacement, which reduces the number of usual suspects.

I see you are planning to fiddling with the linkages. That must might bear fruit, because if the linkage is "too long," it might prevent the MC from retracting fully. Which will do precisely what you are experiencing. In Sophos' case, he pulled out one component of the linkage, and it did not relieve the pressure. But you may be more fortunate.

sjsst41 · 10-07-2011, 08:04 PM

Walt,

I ended up being fortunate.

Yesterday I took the car for a test drive - after 10 minutes - the brakes started to drag. I pulled the brake pedal "back" (no adjustments were made) and this relieved the residual pressure on the brakes (front and rear) I was able to then coast the car with just a nudge.

Again, no adjustment made to the linkage - took the car for a ride tonight with the same results. 10 minutes, brakes drag, pulled the pedal rearward and the pressure was relieved.

I adjusted the linkage (moved the brake pedal in line with the clutch pedal) and also disconnected the fasteners holding the MC in place - and pulled the MC as far forward as I could as well (the movement is small, with the magnitude being the play between fastener stud and thru hole)

Drove it for an hour tonight - no drag or buildup of residual pressure. I believe in my case, the master was not allowed to retract fully.

Stophos · Stophos's Garage

I finally got back to this car. I removed the booster again, looked inside and noticed that the diaphragm seemed to sit higher on one side. I opened it up and reseated it but it didn't solve the problem. I replaced the booster and the problem is solved. I hope this helps someone in the future.

Thanks for the help and insight.

- Chris

sveiks · sveiks's Garage

- rebuilt all 4 calipers with new seals
- new master cylinder
- 4 new rubber brake lines
I did not do a bench bleed of the MC, but I vacuum bled and traditional brake pedal bled system.
After driving for a short time the brakes start to drag, when coasting to a stop the car stops abruptly without using brakes. If I pull back on the brake pedal the brakes release.
I don't believe there is any adjustment between the push rod in the booster that pushes on the MC rod.
Any thoughts here ?
Thanks !

Walt Fricke · 07-17-2014, 11:29 PM

What about adjusting the rod from the pedal to the bellcrank? It sounds like your linkage is causing the piston in the MC to be a bit forward. Doesn't take much movement of the pedal to cause the vacuum boost to start boosting.

I know what you mean about adjusting the actuating rod, though. I tightened up the brake system on my '62 VW once, and when things warmed up the brakes were dragging. There was a play spec for that.