Rear brake pressure reduction valve info

Hi,

I won't go into details about why, but slight, slight possibility my brake problems could be affected by the valve which is supposed to reduce brake pressure to the rear wheels under heavy braking.

Some questions on this:

- is there any way to tell conclusively that it has failed (e.g because when it fails it leaks fluid or something)?
- is it realistic that it might have failed in such a way to constantly reduce the pressure?
- or would it just basically have no affect if it failed, i.e as if it was not there so always normal pressure to rear under any circumstances?
- any idea on replacement cost: are we talking silly money?

No time to do lengthy update on on-going brake problems etc. as forum going down in a few minutes!!!!

Cheers,
Pete