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.