My rear brakes are sticking under hard braking or going down a hill

Hey chaps,

I have an odd (or maybe not) problem with my rear brakes not releasing on my '82 GTI.

I have done a disc rear conversion with Audi 80 callipers, front and rear. Otherwise my booster, cylinders, proportioning valves, etc. are all original GTI items.
Car is lowered on coilovers also, affecting the alignment of the rear proportioning valve.

If I brake gently - no issue.

If I brake hard enough for the rear to rise up a bit, or coming down a hill will do the same, then when I release my foot off the brake the rears are still gripping a little.
As the rear settles down, or I drive off, the brakes release.
If I'm sitting on the flat, stationary, and hard brake, then release, they release immediately.

My best guess is something to do with the rear proportioning valve, which is affected by the angle of the rear axle.
I don't fully understand how that works and I can't find a good diagram or description.
Under tension, does it decrease braking to the rear?

I've read that when doing a rear disc conversion you need to put limiting valves in the brake lines, but I've also read people say you don't.  I have yet to find anyone who can give me a sensible answer based on actually done it themselves.

Any help or pointers would be greatly appreciated.

 

just a guess I would eliminate the load sensing rear proportional valve, and either buy a adjustable one and use that or use the ones off a Cabriolet that are on the master cylinder only.