Rear drum brakes need a 10 PSI residual valve in the line ro prevent air from being pulled past the wheel cylinder cups. I'd suggest putting on a pressure switch with around a 25 PSI rating or slightly more, in the line to the rear brakes. Depending on how vigorously the brakes are applied, line pressure can go to several hundred pounds, maybe even a thousand or more. A Rolls Royce Silber Cloud I worked on once had two camshaft-driven hydraulic pumps for the brake system that operated at 3K PSI! The proprietary brake fluid could only be bought at R-R dealers, and it cost $50.00 a quart!
Jerry
