1972 Rear Brakes Overheating

[stevef1972z]

This problem, for now is the one that is stopping me (actually stopping me too much!) from getting to enjoy the car. 1972 Z stock setup, looks like pre september 1972 construction but not entirely sure of build date yet.

When driving 4-6 miles the rear brakes heat up and expand until there is no ability to coast at all. Here is what has been done to the car so far:

Rear brakes disassembled and cleaned, contact points cleaned and lubed, wheel cylinders disassembled, cleaned and rebuilt no interior pitting, shoes cleaned and checked for proper install, shoes have cut out for adjuster/ebrake to pass through the shoe. Pedal travel measures normal spec. Hand brake operates properly. No leakage, on any wheel. Brakes gravity bled, one man bled and two man bled. Rubber lines replaced at rear wheels. Adjusting mechanism is cleaned lubed and functions with finger pressure and makes good contact with the handbrake lever.

I am seriously confused here! I am not one to go through and randomly replace parts just to get the car moving, the part needs to be proven faulty before it is just replaced that way I know what the problem really was and I am not going to end up in the same boat again.

[MikeW]

Perhaps this is simply stating the obvious but it seems to me like the brakes are dragging slightly even when the brakes are not applied. To me that indicates that they aren't adjusted properly after all and that none of the parts are necessarily faulty.

[stevef1972z]

It is not too bovious. I didnt explain all that I have done to fix the situation. To try and see what the nature of the problem is, I have even adjusted the brakes down to the max. Full free wheelspin with no contact between shoes and drum. My reward was excessive pedal travel, poor braking and about 10 miles before the condition started again heating to the point of hearing, feeling and loss of coasting due to rear brake heat.

I thought shoes as well, but the stamped part number is good, and is the one recommended for the car, as you can tell this is incredibly time consuming, and fustrating. I dont want to compound all of that by just throwing new parts at it that arent needed.

[MikeW]

Can you put the rear up on stands and investigate the brake operation while the car is stationary? Just leave the wheels off and simulate your 10 mile drive. I can't imagine that the distance matters as much as how often and how hard you end up braking. I've got a cheap non-contact IR thermometer that would work well for something like this.

[Seppi72]

There is only one way that brakes heat up - friction. Therefore, regardless of what you might think you have done regarding adjustments, the drums are rubbing against the shoes. If you have freely-spinning wheels when you test them, then it sounds as though the shoes are not fully retracting after the first (or one of the first) brake applications. If the return springs are not broken outright or otherwise "shot," the brake cylinder piston has to be "sticking." Or there could be some piece of crud blocking the brake line at some point so that pressure is not relieved.

Like some of the other guys in our little Z community who have jobs in the techical area, I have a data collection system that works on my laptop. I'd start by attaching a thermocouple to the backside of each of the four shoes, threading the leads through the adjusting holes on the backing plates, and watch the temperatures as I was doing a test drive. That way I could at least tell how many shoes are involved in the problem. Four 0-1" LVDTs would be better, but who has those? Not me.

Once that info was known, I could begin to track down the issue better.

[xray]

Along those same lines, if the rear bias adjuster/proportioning valve malfunctions, it could retain too much pressure after the first or second brake application causing the shoes to stick against the drums...

I've had no success with rebuilt rear brake cylinders--it may be time for new ones.

[stevef1972z]

Turned out to be the ebrake/auto adjuster lever was bent. Fine when brakes adjusted all the way in to get the drum over the pads, but one adjusted, the shoes would hang up on the lever and not return. Need to replace them, looking for them locally first will order online if I have too.

Thanks for the ideas!:classic: