The brake proportioning valve (BPV) controls the relative pressure (and hence the relative stopping power) of the rear brakes versus the fronts. It's designed to increase relative braking in the rear when you've got a load.
It's not a good idea to simply disable it, because this can make your braking behavior wacky. The usual tactic is to relocate it.
Do a search on:
... and you'll find lots of threads where the BPV (and approaches to dealing with it while doing suspension mods ...) had been discussed.