rotor expansion causes rear brakes to lock up?

[91BlckGT]

Hi, as some of you know, I just installed a set of Baer brakes out back of my 91. I've taken it out 4 times now, for rotor seasoning, and gentle heat cycling. The last two times (since I'm starting to drive a little harder and longer), I've noticed that the pedal gradually gets HARDER to push, and the brakes start to groan as I come to a stop.

Now here's the bad part, when I let off the brake, they still groan. And the next time I push the brake pedal, it is even HARDER.

Once the brakes reach a certain temperature, the pads don't stop touching the rotor!!! What this means is, my brakes are not fully releasing the tension, and the longer I drive, the more they grip, and the hotter they get!

Now, my theory is the heat is causing the rotor to expand slightly, but just enough to cause the pads to constantly rub the rotor (causing the friction and the groan).

But is this normal? It shouldn't be. No brake job I've ever done as done this. It's as if the piston wasn't sinking back into the caliper.

Thoughts:

 

[Michael Yount]

I'd park that puppy and not drive it again until I had a long talk with the folks at Baer -- something's definitely not right, and you don't want to take any chances with brakes for obvious reasons. The rear brakes do very little of the work stopping cars like ours as it is - so I'd want to talk to the folks that supplied the kit asap; they've probably come across the issue before.

 

[91BlckGT]

Well, I took another look at it today. And I have a few more ideas.

First off, it's not the parking brake. I thought that there might be some tension on the cable, causing it to be partially engaged. That's not the case, as the little piston that moves when the parking brake actuator is pulled is fully seated, and can't go back any farther.

What I DO think it is, is that I don't have the anchor plate properly centered over the rotor. I even used the .06" shims provided by BAER, but it's still not enough.

So either I re-shim the anchor plates (that's what I'm trying to do right now), or I grind down the pads another .01".

Thing is, with the shims installed, the tolerances on both side of the anchor plate are in spec with what they call for (minimum is .03", maximum is .09")

I seem to recall a thread on corner-carvers talking about having to grind down the pads with the baer rear kit. Maybe this is what they are talking about?

***EDIT***

Actually, I think I might have located the problem. The rotors came with a blue centering ring that slips over the hub. Mine was not flush with the axle flange, but elevated evenly about 1/16" from the flange. After I hammered it on, it seems to fit a little better.

I'm quite certain now, that the groan comes from the OUTER brake pad rubbing against the rotor when the rotor expands, due to the only .03" of clearance between the anchor plate and the rotor. This would be made worse by hard cornering (which causes the rear axle to move left and right).

We'll see today, if my theory holds water.

 

[331 cobra]

I had a '67 that would drag one of the front's. Those old Kelsey Hayes 4 piston calipers had some galling on the cylinders and the piston would hang up in the bore, causing the drag. A caliper rebuild solved the problem.

If your calipers are bad, that may be the cause, but I hope you have already solved the problem.

 

[91BlckGT]

I had a '67 that would drag one of the front's. Those old Kelsey Hayes 4 piston calipers had some galling on the cylinders and the piston would hang up in the bore, causing the drag. A caliper rebuild solved the problem.

If your calipers are bad, that may be the cause, but I hope you have already solved the problem.

It's not the piston, I checked that first. I can pull it in and out of the bore with one hand, no problem. The calipers are brand new.

I just got done with a test drive. Problem still exists. Each time I drive it, it takes a little longer to lock up, but once it happens, I'm fighting the brakes the whole time, since they are dragging. And once they start dragging, it heats them up even futher, which starts a really vicious cycle.

Hopefully baer will have an idea tomorrow. This sucks.