Problem with brakes locking up

et cetera · 10-25-2018, 11:20 AM

I've been having an issue on my 78 SC after overhauling the brake system.

I rebuilt calipers, replaced the rubber lines with SS, put in a new master cylinder and had the brake booster rebuilt. I also put in new pads and rotors.

First time out I didn't notice any problems. Second time I did a bit more driving and once the brakes heated up they completely locked up, as if someone was standing on the brake pedal. I read around the forums for a bit and saw suggestions that you need to "exercise" the pistons in the calipers. So I did that an re-bled the brakes.

Out for a drive, everything is feeling great... Until I brake hard. They release a bit, but it causes the fluid to heat up again to the point which it locks up. I have to walk home and start researching again.

This time I take a look at the brake booster, I want to make sure I'm getting full travel on the pedal mechanisms. Everything looks good there, but I notice that the pin on the booster has a few spacers. I remove those to effectively shorten the pin and give me more travel with the master cylinder.

Out for another drive. Things are feeling good. I'm able to do a good amount of hard braking with out any issue. But eventually things start to lock up again. It releases a bit and I immediately head home, using the brakes as little as possible.

Now I'm just a bit lost. The only other thing I could think of is the master cylinder. I'm wondering if I didnt bleed it well enough and there is some air trapped in there, but I'm not sure that would cause this type of a problem. I'm also wondering if the new MC is just defective. I dunno.

I'd love to hear your opinions/experiences. I went as far as I could with the info I could find on the forums.

pampadori · pampadori's Garage

it is possible to have issue with the fluid flowing back towards the master cyl if the brake lines (both the flexible lines and the hard lines) have a restriction.
But master or calipers could also be to blame. Can you tell if its just one brake that doesn't release or is it all of them? That could tell you where to focus efforts.

Alfasrule · Alfasrule's Garage

Flex lines are bad! Do this 1st, it's usually the cause!

et cetera · 10-25-2018, 12:05 PM

Quote:

Originally Posted by pampadori

it is possible to have issue with the fluid flowing back towards the master cyl if the brake lines (both the flexible lines and the hard lines) have a restriction.
But master or calipers could also be to blame. Can you tell if its just one brake that doesn't release or is it all of them? That could tell you where to focus efforts.

It seems to be all of them locking up. Calipers moved easily when they were exercised, and have no problems until the system is warm.

Its not until there is temperature in the system that things become a problem.

Another reason I dont think its the indivual calipers... After the time I had to walk home I eventually went back to pick up the car. It was facing down a hill, I released the e-brake and took my foot off the brake pedal, car didnt move. I turned the key to the on position but did not start the car. All of a sudden the brakes released and I started rolling forward. I got back on the brake pedal before I hit anything and was able to drive the car home.

To me it feels like the MC is not releasing the pressure.

et cetera · 10-25-2018, 12:06 PM

Quote:

Originally Posted by Alfasrule

Flex lines are bad! Do this 1st, it's usually the cause!

As stated in the post, those were replaced with brand new SS lines.

Stophos · Stophos's Garage

The booster seal is likely bad. I say so because I had the exact same issue on my 88. Took forever to figure it out.

et cetera · 10-25-2018, 01:23 PM

Quote:

Originally Posted by Stophos

The booster seal is likely bad. I say so because I had the exact same issue on my 88. Took forever to figure it out.

How could you tell the seal was bad? I literally just had the thing rebuilt, so I'd hope its not that.

It seems to be functioning fine and travels through its whole range of motion without problems. The pin moves all the way back as soon as you release the pressure from the pedal.

old man neri · 10-25-2018, 02:30 PM

What are you pedal bushings like? I had a bad/rusty bushing causing the pedal to not return fully.

Jdub · 10-25-2018, 02:37 PM

Sounds like your brake pucks & pads are not retracting. In your case either the pucks are binding, or the vent to atmosphere is blocked since you've removed swollen lines and presumably the rebuild work was done by an experienced tech.

et cetera · 10-25-2018, 02:38 PM

Quote:

Originally Posted by old man neri

What are you pedal bushings like? I had a bad/rusty bushing causing the pedal to not return fully.

I had rebuilt the pedal cluster before starting this job and it was working fine. I checked to see that it had full range of motion and everything seems good.

et cetera · 10-25-2018, 02:47 PM

Quote:

Originally Posted by Jdub

Sounds like your brake pucks & pads are not retracting. In your case either the pucks are binding, or the vent to atmosphere is blocked since you've removed swollen lines and presumably the rebuild work was done by an experienced tech.

The pistons retract and move freely during use. When pressure is relived from the hydraulic system they all retract without issue.

What are you referring to when you say "vent to atmosphere"?

Jdub · 10-25-2018, 03:07 PM

I am thinking you would have a vent tube that is plugged, preventing the hydraulic system from essentially resetting to its resting point (that is, pucks retracted and all well). This same effect might be caused by some other functioning part, but this is my best (and cheapest!) guess.

https://www.pelicanparts.com/catalog/images/SuperStock-Priority/911-355-013-13-M100.jpg

But I am a little wary of the rebuild on your brake booster - no evidence at all but is the tech. who did it experienced in this? It's just another item that throws pressure at your system - would be great to compare it to an OEM item.

Best of luck!

Stophos · Stophos's Garage

Quote:

Originally Posted by et cetera

How could you tell the seal was bad? I literally just had the thing rebuilt, so I'd hope its not that.

It seems to be functioning fine and travels through its whole range of motion without problems. The pin moves all the way back as soon as you release the pressure from the pedal.

I chased this problem and did everything including rebuilding the booster and the problem continued. I had a mechanic friend who kept insisting that the booster was bad so Finally i replaced it with a used one and instantly my problem was gone. You may have another issue but many problems are from what was just touched.
One thing you may want to do first is remover the bolt under the dash that goes to the master cylinder and see if your problem disappears.

et cetera · 10-25-2018, 03:50 PM

Quote:

Originally Posted by Jdub

I am thinking you would have a vent tube that is plugged, preventing the hydraulic system from essentially resetting to its resting point (that is, pucks retracted and all well). This same effect might be caused by some other functioning part, but this is my best (and cheapest!) guess.

https://www.pelicanparts.com/catalog/images/SuperStock-Priority/911-355-013-13-M100.jpg

But I am a little wary of the rebuild on your brake booster - no evidence at all but is the tech. who did it experienced in this? It's just another item that throws pressure at your system - would be great to compare it to an OEM item.

Best of luck!

Gotcha. I sent the booster off to power brake exchange for the rebuild. Power Brake Exchange

From what I read they have a good reputation and a long history of quality work.

et cetera · 10-25-2018, 03:57 PM

Quote:

Originally Posted by Stophos

I chased this problem and did everything including rebuilding the booster and the problem continued. I had a mechanic friend who kept insisting that the booster was bad so Finally i replaced it with a used one and instantly my problem was gone. You may have another issue but many problems are from what was just touched.
One thing you may want to do first is remover the bolt under the dash that goes to the booster and see if your problem disappears.

Are you referring to the booster or the master cylinder? The only thing that is bolted under the dash is the master cylinder.

I replaced the master cylinder with a brand new one from our host. https://www.pelicanparts.com/More_Info/91135501112.htm?pn=911-355-011-12-M4

The booster was rebuilt as I couldn't find a source for a new replacement.

Stophos · Stophos's Garage

Yes the master cylinder. Sorry.

Quote:

Originally Posted by et cetera

Are you referring to the booster or the master cylinder? The only thing that is bolted under the dash is the master cylinder.

I replaced the master cylinder with a brand new one from our host. https://www.pelicanparts.com/More_Info/91135501112.htm?pn=911-355-011-12-M4

The booster was rebuilt as I couldn't find a source for a new replacement.

et cetera · 10-30-2018, 08:47 AM

I’m going to try and replicate the problem this weekend to see if I can better troubleshoot things. Just wanted to bump this up and see if anyone else had any ideas/advice.

Evans, Marv · 09:20 AM

This may not be it, but it sounds like the classic case of the space between the brake pedal rod and the master cylinder not being adjusted correctly. When the pedal rod & piston contact on the master cylinder's piston are too close, vibration of the pedal will cause the rod to barely vibrate against the contact face of the master cylinder piston. This causes pressure to be built up in the brake system which causes the brakes to bind up after a short time. I forget what the amount of free space should be (3mm comes to mind, but it's been so long, I'm not at all sure), so check out the free play from when the pedal starts being actuated and when it makes contact with the master cylinder piston. If I remember correctly there is a double nut on the rod to set the adjustment.

Jonny042 · Jonny042's Garage

Quote:

Originally Posted by et cetera

The pistons retract and move freely during use. When pressure is relived from the hydraulic system they all retract without issue.

To me, this says that pressure is building up in the hydraulic portion of the system (of course). The only way this can happen is if the fill/return holes are being covered by the master cylinder umbrella seals.

As the master cylinder is pushed in to apply the brakes the pistons immediately cover the fill/return hole and only then can pressure start to build.

Something is causing the master cylinder to be slightly activated at all times. So it could be something "off" with the booster, no play in the linkage, etc.

With the bleeders closed you should be able to push the pistons in and fluid will freely flow back up into the reservoir. If this isn't happening there's only one thing it can be - the holes are covered.

Maybe next time you get the system "pressurized" you could try loosening the nuts that hold the master cylinder to the booster. If that releases the pressure you know there's something wrong with the clearances between the booster and cylinder or somewhere else in the linkage.

Instead of driving and trying to heat things up I think you might be able to push a piston against the fluid pressure in one of the calipers and it will have the same effect as heating up the brakes. If the fill/return hole is indeed covered the other brake calipers will apply pressure on their pads so make sure there is a rotor between the pads!

Perhaps a safer way of going about this!

et cetera · 10-30-2018, 03:12 PM

Right now I have the brake booster push rod shortened as much as it will go and the pedal has a good 10mm of "play" before it engages the MC.

Is it possible for air in the MC to cause this type of behavior? It really seems like its just having difficulty releasing the pistons in the MC and allowing fluid into the reservoir.