Disc Brake Hold Off & Prop Valves

[QUOTE
However I also tend to believe they help in keeping air out of the system which could enter at the wheel cylinders or calipers.[/QUOTE]

,,,don't you think that if air could leak in, brake fluid could leak out,, especially at the 700 to 1400 psi at the wheel cylinder - caliper with the brakes engaged,,

My 68 Barracuda was a 4 wheel drum car, converted to 4 wheel disks, used the 73 and up mopar A body disks / calipers on the front, a Dr Diff disk kit on the back, using a 68 A body distribution block in the brake line system, no proportioning valve, no hold off valve,, stops fine.