Need help with brake problem

[ttw18]

Hello, finally got my project on the road (pics to follow soon). I have taken a few short shake down trips and have encountered a brake problem.

Here is some info: per previous owner
rebuilt M calipers
new master cylinder
new pads, fluid etc.

Here is what happened:
When I went on a longer trip and the brakes heated up more, they began to gradually bind until the car was unmovable. When I came back to get the car and they had cooled off and I was able to drive it again.

So I:
Had to hammer off the front caliper
Removed the calipers to inspect and the piston moves freely.
Noticed pins centering pads had no grease.

My theory:
The pins prevented the pads from retracting as things started to heat up and expand. This explains why they were able to work again when they cooled off.

Any thoughts?

[rllevin]

Old rubber brake lines preventing return of fluid from pistons?

[ttw18]

nope new brake lines.

[DRACO A5OG]

Are you absolutely certain the Spring is placed properly?

It sounds like the springs are not puting enough tension on the pads to keep them away from your rotors when pressure is released.

Placing anti sieze on the pins and spring where the back top of the pads meet the spring is a must but even if you forgot or did not have the antisieze the spring (if clean) should give enough tension to move the pads out when pressure is released.

my 2 cents

[ttw18]

The spring was installed correctly as far as I could tell. Still confused as to why the calipers/pads not relasing when hot.

[Flieger]

I know you said new flex lines, but it sure sounds like swollen rubber flex lines to me.

[DRACO A5OG]

How about some pics Brother?

[barney911rs]

Both fronts were stuck or just one? I agree with Flieger, it does sound like bad lines. Get some new OEM rubber lines and see if the problem goes away. One way to test this, is when they are "stuck" crack open the bleeder and see if that releases the pressure and the pads will retract. Be careful not to spray the fluid on hot brake parts, as it could catch fire.

[ttw18]

2 more things to add:

Seemed like driver front caliper was the hottest.

The pedal became stiff when brakes started to bind.

[rusnak]

do you by chance use a motiv pressure brake bleeder?

[ttw18]

I have one, yes. I have not used it on this car yet.

[Flieger]

Just throwing this out there:

Knock-back pin mechanism binding when warm?

[rusnak]

I was thinking maybe the vent tube is still clamped or pinched in some way.

[ttw18]

Flieger - Not sure what you mean "knock back pin" I am coming from a bmw background; same language, different terminology

Rusnak- vent tube? are you talking about the motiv?

Keep it coming guys! I appreciate it. I think it is the last thing keeping my car off the road.

[rusnak]

no, the master cylinder reservoir vent tube. Crazy theory, probably far fetched, but what if someone forgot to release the tube clamp, and the brake fluid got hot, and the air in the reservoir started to expand?

[Flieger]

I posted here before rusnak but it didn't work.

I have a tube that goes from my fluid tank vent through the smugglers' box and out the bottom of the car. Now the trunk is not wet anymore and there is blue fluid in the lines and on the ground. I guess I just go too fast. I don't have any interior in the cabin or trunk but it keeps the paint from melting.

The knock-back mechanism is a pin in inside the cylinder bore mounted to the outside cover of the caliper body. The piston has a tube protrusion with a brass tube and a spring that grabs the pin and lightly forces the piston back toward the rotor. This makes the piston react quicker and build up pressure faster:
http://www.early911sregistry.org/forum/showthread.php?t=17846
I know these are S calipers but Ms have them too. See the M caliper thread: http://www.early911sregistry.org/forum/showthread.php?t=16254

If something is expanding when warm and binding, it may not allow the piston pressure to relax.

Just a thought.

[ttw18]

I think I made a mistake. The calipers are on my '67. I thought they were called
M, I was wrong.

[Flieger]

L calipers?
http://www.early911sregistry.org/forum/showthread.php?t=20424

[barney911rs]

on a 67, that eliminates a brake boster problem, so the clogged MC vent is very possible. That's why I suggested cracking open the bleeder next time it's stuck. If the pads release, it's not a caliper problem.

[ttw18]

Thanks guys,

I am going to check the MC vent line this weekend. I also am going to replace the rear lines as they may be old contrary to my previous belief. The previous owner may have only replaced the fronts.