According to haynes it is a pressure reduction valve like old cars would've had which reduces pressure to the rears under heavy braking to prevent the rears locking and keep the bias to the front (where all the effective load/weight is).
I should stress that for my problem it is most likely that this is not at fault, but it might be so I wanted to get a potential idea of what to look for and cost.
My problem is that under anything approaching heavy braking the ABS activates in such a way that the pedal gives and stays like that, i.e the ABS thinks one or more wheels are decelerating too fast and then even when the ABS blocks the brake line or pumps back the fluid then it stays like that. I have disabled the ABS and I can tell that the rears are braking too much with respect to the front.
Now, I think the problem is actually a fricition problem with one or both rear brakes not so much binding all the time but maybe seized caliper guide pins forcing one pad onto the wheel or a knackered caliper or something that causes one or both rears to brake way too much. BTW previous symptoms suggest I also had front brake problems but for sure this problem is now a rear brake problem. I should also point out I strongly suspect the ABS is not at fault.
The brake pressure regulator COULD be at fault on one or both circuits if it was, in some circumstances only maybe, failing to reduce the brake pressure to one or both rear brakes.
Hope it kind of makes sense,
Pete
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