Brakes sticking.

[cevallos.j]

Hey guys, so I am having an issue on our 1980 targa. I picked the car up with the engine apart so while the engine was getting rebuilt we rebuilt the calipers. Now we put the whole car back together and got it running and after driving around for a bit the front brakes start to stick. Pretty badly. Took the calipers off to double check they were built correctly and seems they are, and the piston comes in and out fine manually. I can blow in the lines and the fluid flows out fine, kind of lost on what else it can be. Seems to be after about a 10 minute drive, then I let it sit for a bit and it releases. Also the car has a new master cylinder.

Any Ideas?

[pa911]

Did you replace the brake hoses? They swell up on the inside and act like a one way valve.

[Trackrash]

+1

[cevallos.j]

Will try that and report back. There's a few threads with similar problems as mine but no one actually posted back with what solved it.

[frankc]

Since it is both front brakes I suspect what is happening is that the master cylinder pushrod is not retracting fully, thus not allowing the brake fluid, that is expanding due to heat, to dump back up into the reservoir as it is supposed to when the brakes are released - this happened to me once after rebuilding a 911.

The sequence is that everything is fine until you use the brakes for the first time, the fluid is then heated and expands slightly. But since the expanded fluid has no where to go when you release the brakes, the fluid pushes the pads out a little causing them to drag on the calipers, which causes the fluid to heat up and expand even more (without ever touching the brake pedal), thus causing the fluid to apply even more pressure on the pistons & pads, and the whole thing snowballs from there. If you let the car sit for a short time, the fluid cools and retracts, and everything is back to normal.

Try this: when the brakes start to drag, reach down and pull up hard on the brake pedal to see if that releases the pressure - that worked for me. If this works, then you need to adjust your brake linkage. I eventually ended up replacing my master cylinder as the problem would return occasionally even after adjusting the linkage, and I suspected that there was dried brake fluid internally due to the MC sitting with the fluid drained for many years (plus it was leaking). That did finally resolve the issue for me.

[cevallos.j]

So changing the brake lines was pretty horrible, the threads all stripped and it was difficult to removed but we managed! Unfortunately still having the issue.

Frank that makes a lot of sense, I will play around with that tomorrow.

[cevallos.j]

So I think I am halfway there. Frank you are right, if I grab the brake pedal and pull back the brakes free up. Is the linkage you are talking about directly behind the brake pedal? I played around with it but it didnt help. The master cylinder doesnt retract fully when I go off the brakes. If I look into the brake booster while someone pulls back on the brake pedal i can see it release the master cylinder fully and the brakes free up.

So I guess my question now is, what can cause my master cylinder to not retract fully? is there any adjustment inside the brake booster or anywhere other than behind the brake pedal? The master cylinder is brand new.

thanks guys

[frankc]

Hi Jorge,
Since you have a power brake car, the linkage to adjust is the bar that attaches to the brake pedal and then goes up and attaches to the brake booster pivot. Try adjusting that rod shorter such that there is no pressure on rod when brake pedal is at rest.

You will need to loosen the lock nut on both ends, then turn the rod in the direction to shorten it until the rod feels loose, and the tighten the lock nuts.

In my case, I had installed a new rubber bumper for the brake pedal when I rebuilt the pedal cluster, and this caused the pedal to rest at a slightly higher position - just enough to prevent the master cylinder pushrod port from opening.

[frankc]

After reading your post again, what is the condition of your pedal cluster? Are you sure that the brake pedal is returning all the way to the rubber stop on its own? If not, then you may need to replace the bushings in the cluster.

[cevallos.j]

Frank, when its all disconnected the brake pedal does go all the way to the end on its own. When I pump the brakes, then pull them back with my hand, the pivot goes back an additional centimeter causing the brakes to unlock. I took it all apart and in pieces they all appear to be working. Master cylinder retracts, brake booster retracts. Almost thinking of adding a washer or two in between the brake booster and master cylinder to work as a spacer and help to relieve the master cylinder of pressure.

When I put the master cylinder into the brake booster outside of the car, the master cylinder piston is already being pushed in about 1 cm. is that normal? I assume it is because same happens my spare master cylinder.

[ganun]

There should be a small (2mm) gap between the booster rod/pin and the master cyl piston. On some boosters the protrusion of the rod is adjustable to give this gap.
Or your booster could be rusty inside and hanging up, look inside the vacuum port.

[cevallos.j]

So the problem was in the brake booster, It wouldn't retract all the way so it wasn't allowing the master cylinder to relieve all the pressure.

All is well now

Thanks to everyone!