Disc Brake Hold Off & Prop Valves

Residual pressure valves are only to keep the shoes and pads near the drum or disc.
Drums use a 10lbs valve and discs a 2lbs valve.
They shorten brakepedal travel by not having to take up the gap between brake shoe and drum, or discrotor and pads.

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