Rear brakes overheating

[Blaze0010]

Now with the car road legal, i'm doing some test run to check that everything works fine.
On the engine side everything is good, gearbox so and so and might need a syncro gear replacement (3 to 2 is kinda hard), the diff is quite noisy but most likely due to old C&P and probably also not perfectly aligned, but i will address it soon.
The main problem i'm facing is that the rear brakes overheat and lock up. If you then wait 30-40 minutes, the discs cool down and you can get back on the road.
It happened after 3km with a lot of stop and go due to traffic.
With almost no traffic at all i can drive for more than 30km before it start to overheat.
I'm trying to understand what could the cause be but i gladly accept suggestions.
What i've come up so far is:

- Glazed brakes pads, which require more force to stop the discs and causing more heat, leading to a quick overheat.
- Faulty proportioning valve which not let oil get back and keep pressure.
- Overheating diff the overheat discs (it will probably explode before reaching that kind of temperature)
- Faulty brake calipers which doesn't goes back.
- Overheated master cylinder, but probably would lock up also the front.

The weird thing is that once everything cool down, it starts to run normally again, and this would make me think that cannot be related to neither the calipers or the proportioning valve.

[Dave W]

I had the similar problem. The rear brake caliper could not slide freely, allowing the pad to retract, and/or rust in the cylinder bore. Which caused dragging and overheating.
Caliper rebuild kits are dirt cheap. While apart, polish the piston and cylinder bore, and coat with brake piston with fluid and then lube all the sliding surfaces.
Buy enough brake fluid to bleed all the calipers and get fresh fluid in all the calipers.

[Blaze0010]

(10-03-2025 01:00 PM)Dave W Wrote:  I had the similar problem. The rear brake caliper could not slide freely, allowing the pad to retract, and/or rust in the cylinder bore. Which caused dragging and overheating.
Caliper rebuild kits are dirt cheap. While apart, polish the piston and cylinder bore, and coat with brake piston with fluid and then lube all the sliding surfaces.
Buy enough brake fluid to bleed all the calipers and get fresh fluid in all the calipers.

Problem is, rear caliper are brand new from rockauto (regenerated), so can't be that.
One thing i've noticed it's that is possible that also front brakes have a slight drag, but managed high temperature better than rears.
So i'm currently cleaning the master cylinder and try to make the correct adjustment to the brake booster push rod.
Since the issue seems to be affecting all brakes (more the rear) maybe the push rod has not the correct tolerance and generate a little bit of drag, keeping the brake even so slightly pushed (or not letting the piston get back) enough to overheat the discs.

[banpei]

As it slowly and gradually happens and after a cool down is fine to drive, isn't there another source of heat that could make the brake fluid expand? Can you check if your bearings aren't overheating? Or maybe one of the brake lines is too close to the exhaust?