The references to "m/c under the floor" or "m/c on the firewall" is misleading. I assume they are talking about lowered street rods. The important thing is this:
If your master cylinder is below your calipers then a 2 lb. residual valve (RPV2) is needed to prevent fluid from flowing back from the calipers into the master cylinder.
In my case the m/c is under the floor. But since it's a truck at close to normal ride height, the m/c is above the calipers and thus no RPV is required.
I have Camaro disk brakes, Camaro rear drum brakes and Camaro m/c with the combination valve. There was no rear proportioning valve in the car and one is not needed tor the truck. Camaros were front heavy, too.
