I've had this before on a bike, had two problems, disc was out of true, and brake fluid was old, so when moving, disc rubbing on pads warmed the caliper/brake fluid, once it got warm enough the moisture in the fluid (brake fluid is hydroscopic, hence regular changing), changed to steam locking the brakes on, when cool, brakes were back to normal. New disc and a thorough fluid change solved the problem.

They were all new parts we put on, even tried 2 New calipers as we thought the 1st one was faulty. Been to put some fuel in this morning & the brakes were fine it's just when they start to warm up it gets tighter & tighter (the piston is sticking, but as soon as you loosen the nipple it retracts back with ease)

 

Have you bled the whole system with fresh fluid yet? I would be in the same boat as the last poster, #26, you might not have a warped disc but still the oil overheating causing expansion.

Yes, we put fresh fluid in when we put a new master cylinder on. I've left it with a friend who is a mechanic now, hopefully he will solve the issue. Will let all know what the problem was once its fixed