The purpose of the residual check valve, (Figure
46-13) found only on vehicles with rear drum brakes, is
keep light (or residual) pressure in the brake lines and
also at the wheel cylinders. Without residual pressure, air
can be sucked past the wheel cylinder cups and into the
wheel cylinders. This could occur when the brake pedal
isreleased quickly. Residual checkvalves are designed to
permit hydraulic fluid and pressure to flow from the
:rnaster cylinder into the hydraulic lines when the presin
the master cylinder becomes great enough to
'open the valve. After allowing the fluid to flow back into
ithe master cylinder when the pedal is released, the outlet
checkvalve spring (Figure ffi-14) takes over and keeps
the hydraulic lines and wheel cylinders full of fluid, ready
for the next application of the brakes.