Seeking ideas concerning what may be causing the rear brakes to drag and how to resolve the problem

Hi Lou -

I wasn't sure where to put this comment, but since this is YOUR thread, it might go here...

Anyway - if your grounding (such that the temp gauge isn't reading correctly) is affected by paint and goes through the strap from your ZF to the frame, then you may (or may not) eventually have starting issues.

I might suggest you run a heavy ground cable from one of the starter attachment bolts to a solid ground on the chassis (mine goes up to where the engine bay brace attaches).

It is a $10 expenditure that is an easy mod, and in my opinion, you can never have too many solid grounds in your vehicle...  It makes your electricals more robust.

Good luck on the brake issue - that has me puzzled.  Did the brakes release when the car cooled off?

Rocky

Hey Rocky,

The car seems to roll now that the brake system has cooled. I had some heat wrap laying around so I wrapped the master cylinder just as a extra precaution. 

Some day I'll probably install some self adhesive heat shield that I can carpet over. Currently the front trunk in my car is just painted sheet metal so it makes sense to me with the fans blowing hot air right at the front trunk and no carpet or type of insulation it must get really hot in there if the car is just sitting or in stop & go traffic.

Louie

rocky posted:

, , , on the brake issue - that has me puzzled.  , , ,

 

that has me puzzled too.   Is there any check valves in the master cylinder?   I didn't "think" so.   I thought on pedal release the fluid in the lines is ported back to the resevoirs.

A far fetch fault I could imageine is thermal expainsion of the booster is "pressing" the master cylinder's pistons.   But the recomended clearance setting between to two at off pedal is 0.035".   just a gut evaluation, I would not expected more than 0.004" thermal growth could be possible.

I seriously doubt your front trunk is getting hot enough to thermally expand the brake fluid alone. Did you check bind was only rear brakes? Both sides? It may be as JFB suggested the push rod length tolerance being too tight. Also it was quite common for aftermarket brake master cylinder adapters to have a plastic guide for the push rod, these are known to be too tight a tolerance and bind when they get hot and need reaming out. If rear only I'd also check flexible line routing near the exhaust, presumably you have changed out the old rubber flex lines for braided hose?

Julian

Same brakes have been in my car for years and the car does get driven once a month if not more. I never measured the push rod clearance so I may check that out. There is a chance the brake cable for the rear passenger side got hung up under the trans when I jacked up the car causing the pads to drag which created a heated caliper and heated fluid. Car rolls easily now but I’ll need to wait a few weeks before I can go for another drive. Thanks for everyone’s advice!

joules posted:

I seriously doubt your front trunk is getting hot enough to thermally expand the brake fluid alone. Did you check bind was only rear brakes? Both sides? It may be as JFB suggested the push rod length tolerance being too tight. Also it was quite common for aftermarket brake master cylinder adapters to have a plastic guide for the push rod, these are known to be too tight a tolerance and bind when they get hot and need reaming out. If rear only I'd also check flexible line routing near the exhaust, presumably you have changed out the old rubber flex lines for braided hose?

Julian

 I had exactly the problem Julian describes with aftermarket master cylinder and adaptor, when I bought my car. Rather than messing around reeming out the adaptor etct, getting the proper mastercylinder, with no adapter needed solved the problem immediately and it hasn't returned after at least 4 years. However recently I noticed the back brakes hanging and it turned out the parking brake cable had seized inside the casings, stopping the brakes from slacking off. 

Std caliper o-ring seals are the only thing that causes brake pistons to move away from the rotors after use, plus of course any disc runout. When braking, the o-rings distort under hydraulic pressure. Brand new, the pistons only move about 0.010". I suppose really old o-rings would lose some resilency but I'd also expect to see fluid leaks as well if it was old o-rings.

The first time this came up, it was caused by seizure of the e-brake cables inside the cable jackets between the under-car bellcrank and rear calipers. Removal plus soaking in thin oil for a few days fixed the problem.

The second time (GT5-S), it was caused by having the parking brake on when someone undercoated the car.....It destroyed the tiny late e-brake pads within a few miles coming home.  Both were OEM cables and 40+ years of weather seems more than enough time to rust such things once the plastic jacket cracks.