In some brake systems, especially those with drum brakes, a small component called a residual pressure valve does a quiet but useful job. Here is what a residual pressure valve is and why it exists.
What it does
A residual pressure valve holds a small amount of pressure (typically a couple of psi) in the brake line to the drum brakes even when your foot is off the pedal. This keeps the wheel-cylinder cups lightly seated so they do not leak or let air in; see brake hard lines.
Why drum brakes need it
Drum brake wheel cylinders rely on the shoe return springs to retract - without a little residual pressure, the cups can relax too far and allow air in or seep fluid. The valve keeps them primed for a quick, firm response.
Discs are different
Disc brakes generally do NOT want residual pressure - it would keep the pads lightly dragging. That is why disc systems use a different (or no) residual valve. Mixing this up in a drum-to-disc conversion causes dragging brakes.
When it matters
Residual pressure valves matter mostly during brake system work and conversions - use the correct valve for drum vs disc. Keep the whole system healthy with fresh fluid and quality pads and hardware.