I have a strange one I haven't come across before.

Current setup

Willwood pedal box with Bias bar

Front Willwood minilites with solid discs

Rear standard sierra discs

When i brake hard the rear callipers are binding on.

check the master cylinder and there is play in the plunger so not holding pressure.

If I operate the handbrake and then release it the wheels free off again.

First notion was faulty handbrake mechanism but it happens on both sides, scratching my head.

Just started doing it?

As you've had it on the road a fair while to be just noticing this now?

Have you checked the handbrake arms are returning fully on the calipers once the lever is lowered?

[Edited on 8/7/17 by CosKev3]

Yeah just started at last track day

Were the rear calipers a matching pair when fitted? (Both refurbished by same place etc)

As I'm with you seeing as pulling the handbrake on and releasing then releases the wheels,both calipers faulty?

Both Callipers were bought together new