Re: Problems with brakes on -99 GTS ACR, same system on all GTS?
That thing is called the Brake Proportioning Valve. It is designed to limit the rear brake pressure once you reach 800 psi (if memory serves). One thing many of us do is actually disable that assembly by either removing the 0-ring seal, removing the spring, or the plunger, or some combination. Basically, you want to disable that plunger from rationing the rear brakes.
So rather than find a new part, you can likely remove the part thats getting stuck all together and be even better than new. Do a search for "prop valve" and you'll find lots on it.