Brake Problems

Hi, I have had problems with the rear brakes binding after running aporox 5 miles or more, rear wheels are too hot to touch, once everything cools down you can drive again for a similar distance.
I have replaced the rear calippers twice with brand new ones, thinking the original ones were knackered, bought a brand new pair, genuine old stock, and again they seize once hot, bought another brand new pair and still the same happening.
I upgraded the front with the big brake kit. I have replaced all the flexible pipes with stainless steel braided ones, I have replaced the servo and master cylinder with a 2nd hand one, I replaced the brake balance valve.
Handbrake is slackened right off.
Still the rear brakes are binding once you've run 5 miles or more. Let everything cool down and all seems fine again.
Has anyone experienced this with fitting the big brake kit?
The MGTF doesn't have ABS so it's a straight forward system but yet its still happening.
I checked the servo vacuum pipe and that's all in order.
Pulling my hair out now trying to figure out what the problem is.
Any help greatly appreciated.

That’s an interesting one, with what you’ve done you should have the best brakes of any example of your car.
My guess is your master cylinder is pumping the system up a small increment at each pedal stroke & not allowing that pumped fluid back into the reservoir until everything clamps up. Let it cool & as it cools the pressure slowly equalised, then your ready to start the cycle again.
I only attend to problems I come across on my own vehicles & thus my in-depth knowledge is limited to those experiences, brake master cylinders haven’t troubled me & so I’m not as familiar as in other areas. I am conscious of the push rod which carries the seals not fully returning & thus not clearing one of the fluid ways in the outer casting, & not allowing the pressure to equalise.
Thats the only thought I have at this time in the morning!
I’m sure the rest of the lads will have more ideas but do let us know how you fare.
M

Is this happening to both rear brakes or only on 1 side ?

If 1 side only it would be most likely to be a defective flexible hose; In rare instances the inner hose can break down forming a flap which blocks the fluid returning back towards the master cylinder.

If it's both sides I would strongly suspect a fault on the master cylinder for the reasons Airportable has given. Being a dual circuit system its quite feasible to have a fault on one circuit whilst the other continues to work ok for the front brakes.

If the previous suggestions don't give a solution, then a couple of odd ball ideas. Have you fully wound the brake piston back in before fitting the callipers? When putting my own big brake on I found that the pads rubbed on the discs despite the pistons being right in, on checking the dimensions the pads were each one mm thicker than virtually all the other brands.

It's both rears, servo and master cylinder were replaced by 2nd hand items. I also replaced the brake balance valve with a 2nd hand one and still the problem persists.

I did think the new pads along with the new discs were making the clearances tighter. I am going to take a 1mm off each pad and give it a try.

That seems a bit extreme.
Out of interest, have you recently replaced the rear bearings? If they are not fully torqued up, the discs are misaligned to the calipers and will cause binding.
Or is there any chance that the handbrake cable needs some adjustment to back it off?
I recently replaced both rear calipers with new ones, and had to back off the cable.

Did the problem with the rear brakes exist before you changed the master cylinder ?

Yes the problem happened before I started to replace everything. The first time it happened my TF had came out of hibernation and I had driven approx 10 miles and both the rear brake seized. I thought first of all it was the callipers as they were still the original ones. I had already purchased the big brake conversion kit so when I fitted a new set of rear callipers, new rear discs and new rear pads I fitted the big brake kit to the front. Bled everything and all was fine for the small test drive I did, then due to other things going on the TF was never used again until the following year for the MOT and it sailed through the brake test.
I took part in a classic country run and after approx 15 miles the rear brakes seized on again. Thankfully the run had a recovery vehicle following so I ended up on the back of the recovery. By the time we got back to the show ground the brakes had cooled and the car was free to move again.