brake trouble, front brakes lock when hot Options

[trfrick]

I need some help - I replace the front rotors, calipers, pad, flex line and m/c. Now when I went for a test drive after several hard stops the front brakes get hot and lock up.
After they cool they release.

What should look for?

Thanks
Tom

[jcd914]

I would start by checking that the brake pedal is letting the master cylinder come all the way back. There needs to be a little clearance between the push rod on the pedal and the piston in the master cylinder. And make sure the brake pedal moves freely, not binding on the bushings.

Jim

[McMark]

Good call, Jim! (IMG:style_emoticons/default/thumb3d.gif)

[jcd914]

Experience is a teacher you remember!

[ldsgeek]

Experience is a teacher you remember!

Yeah, unfortunately she gives the test first and the lesson after (IMG:style_emoticons/default/dry.gif)

[trfrick]

I would start by checking that the brake pedal is letting the master cylinder come all the way back. There needs to be a little clearance between the push rod on the pedal and the piston in the master cylinder. And make sure the brake pedal moves freely, not binding on the bushings.

Jim

The pedal is adjusted correctly - when the brakes heat up the free play goes away.

[Eric_Shea]

Where'd ya get the crappy rebuilt calipers? (IMG:style_emoticons/default/hide.gif) (IMG:style_emoticons/default/biggrin.gif)

Tom, can we determine if a piston is sticking or not? Short drive, does one caliper get hotter than the other. Short stop, hands off the wheel, does it pull to one side?

It almost sounds like one of your knockback mechanisms' is not working properly causing excessive heat to the system, especially if you have new lines and MC.

Do you see any "blueing" on one of the rotors vs. the other?

Otherwise I'd double check that pedal free play again as it sounds tailor made to your problem.

One other thing to check is the bearing play on your new rotors etc. Double check the spindle nut for tightness and make sure there's no excessive runout causing the rotor to wobble and contact the pads causing heat.

Check the pad gap with the car cold... both inside and outside. Can you turn the rotor by hand (when cold) or do you feel a substantial amount of rubbing? If there is substantial rubbing, we may want to look at the bearings as a possible cause.

Let's dig in...

[trfrick]

Thanks Eric - I don't think it is the person who rebuilt the calipers as much as the one who installed them (IMG:style_emoticons/default/blink.gif)

It does not pull to one side but the right does seem hotter than the left when it happens.

WQhen its cold I can turn the wheels buit there is some rubbing, a little more than I expect.

What should the pad gap be?

I will double check the spindle nut.

Thanks again!

[r_towle]

It almost sounds like one of your knockback mechanisms' is not working properly

can you explain (with pics if you have them) what this mechanism is on the front calipers?

I have the same problem on my 911 and I have not replaced everything...so its probably the rubber lines for me.
Just curious ....never seen a specific thing in the fronts that would do this.

Rich

[Eric_Shea]

can you explain (with pics if you have them) what this mechanism is on the front calipers?

I'll grab PDF's from the factory manual to show everyone when I get to the office.

Think of the toy monkey that ratchet's its' way down the pole only these work in reverse. As your pads wear, these ratchet the piston out toward the rotor. There's a spring in there as well and they're designed to keep a high pedal feel. You'll probably recall seeing a pin in the piston bore or a mechanism on the backside of the piston.

On very early calipers (356, 911 M, S and rear M) these mechanisms were clipped into the piston with a fairly standard c-clip. 914 calipers and later M-Calipers came with this mechanism machine pressed into the piston. All 914 front caliper pistons go through an ultra sonic bath to clean the mechanism because they will not seperate from the piston. Even later calipers (1975 911 front and rear and SC/Carrera) had this mechanism removed and a "cup" type piston was used. This offered slightly better cooling as there was more surface area in the cup style.

Anyway... I'm more concerned now that there is what I would consider significant rubbing. Tom, is it rubbing on one pad set vs. the other; specifically inside vs. outside? Gap should be around .004" or thereabouts. Again, it's laid out in the manual in the same section but, I'd like to look at it before spreading mis-info.

[Eric_Shea]

Here's the file... the clearance number I spewed earlier is within spec.

 Knockback_Description.pdf ( 228.28k ) Number of downloads: 119

[trfrick]

OK - I have no clearance between either pad and the disc.

[Eric_Shea]

That can be normal for the first few miles on new pads. Pull the pads and check that the rotor is centered in the gap. If not we need to look at bearings. If so, I'll start prepping some new pistons.

[r_towle]

cool,

So this knock back device....it rackets up as the pads wear so you would need to add fluid over time to fill the void behind the piston...

What and how do you reset them once you ahve new pads? Just push?

Rich

[Eric_Shea]

Just push?

Si... we press pistons back in with an Arbor press. We also look for the "bounce" when they've reached bottom. The pistons will usually bounce back about 1/16th of an inch showing that the mechanism is working properly.

[trfrick]

I loosend the spindle nut as you said and reinstalled the pads. While the pads were out everything appears centered. But the outside piston is sticking out further than the inside.

When I went for a test ride the same thing happend. Everthing is good for a feww low speed stops - then after a hard stop the free play on the pedal gradually disappears and the brakes lock (both sides)..

[toolguy]

The bad part is you replaced a whole bunch of stuff, any one of which can cause problems . . . It sure sounds like you don't have pedal freeplay . . are all the calipers and the MC new? or did someone rebuild them ??
are you sure you're starting with known good units ?? If you're not sure, it's like shooting yourself in the foot. . .

Lets get to basics first for trouble shooting,

When the master cylinder is in the released or at rest position, you should be able to pry back each brake pads. . . the fluid you're compressing in the calipers goes back thru the uncovered inlet port in the master cylinder up to the fluid reservoir. . . If you can do this, then there is no mechanical reason why the pads when the pedal is released can't do it themselves. but your statement about only when hot or cold makes me think something is expanding when hot and binding. .
If you cant push the any of the pads in , my first guess is the master cylinder probably has the rod depressed and the piston is past the port hole for whatever reason.

When it's working correctly, as you step on the brake, the MC piston covers the inlet port, sealing off the reservoir, and forces fluid out to the calipers. . not back up to the reservoir, making it a one way system, but only if the piston port is covered.

If you can pry back any one, then the MC port is uncovered. . . . . Next, Make sure your can pry all pistons back. . . it should take only light force. . . you may have sticking pistons but I can't imagine all 4 sticking at once . .
Do all the pads fit in the calipers easily, do the move freely ? or do the rub and bind on the caliper ??

If you can move all the pistons, you might try putting your old pads back in a see if the problem changes. . that's easy. . . If that doesn't do anything and you're still puzzled, put the old MC back in. . that's a pain ion the butt, but there isn't anything else left to change. .

Just remember, you're going to solve the problem, it's just a matter of the steps you have to go through to get there. .

[trfrick]

The MC was replaced about two months prior to the rest of the parts and was working well.

I have plenty of free play/ with the brake pedal until the brakes heat up the it goes away. Shouldn't the excess fluid go back to the reservoir?

I plan on changing the fluid again to see if any "crap" comes out since it does not appear the brakes were well maintained over the last 40 years.

[PRS914-6]

Is the vent on the M/C reservoir plugged ? There is a vent and if plugged or capped would give symptoms like that

[toolguy]

The MC was replaced about two months prior to the rest of the parts and was working well.

I have plenty of free play/ with the brake pedal until the brakes heat up the it goes away. Shouldn't the excess fluid go back to the reservoir?

Yes, but only if the MC port is uncovered. . . if the port is covered, then the brakes get hot, warm the brake fluid, the fluid expands it has no where to go so it pushed the MC piston back and that takes up all the freeplay. . when the freeplay is gone the ever expanding fluid forces the caliper pistons out and the brakes press even harder on the rotors, they get hotter. . Get the loop your in ?? it just gets worse till the wheel lock up. .

Go back to the "see if you can pry back a pad" thing. (IMG:style_emoticons/default/confused24.gif)
. you should / you want to see the fluid rise in the reservoir . that's the only sure sign the port is uncovered because prying back one pad will also force the other front pads to expand. . . make sure the fluid really is going to the reservoir. .

So the only thing new is the calipers and pads?? So are the calipers direct replacements, or did you do some kind of swap or upgrade ?? If the MC was working fine before, it would seem something is amiss in the wheels. . but both at the same time is questionable. still seems like a MC problem from afar. .

Doesn't your master cylinder have a warning sensor, screwed into the side with a couple of brown wires?? those wires go to the idiot light on the dash i.e. same light as the handbrake warning light. . . that's also the warning light that the piston is not in the middle of the master cylinder on dual circuit master cylinders , hence a problem / leak in the front or rear systems. . .

Another item to look at, If the brakes are getting hot enough to lock the wheels over and over, I gotta think you've got all kinds of air in the calipers by now. .