Residual check valves came in master cylinders to provide a small amount of pressure(~10psi) to the wheel cylinders while they were not applied, this kept them from seeping
and prevented them from taking in air & moisture past the flared cup seals. This would occur esp. when they were heated up good, then parked to cool...the colder the worse.
At some point, the spring & spreader discs arrangement was added to the wheel cylinders, these contact the inner part of the tapered seals and kept them flared snug so that
the MC's no longer needed a residual valve. Only the large trucks w/the tandem MC's still contained a RCV for the rear drum brakes.
Metering valves are provided in a Disc/Drum setup to maintain ~3-30psi to the discs until the gap between the rear shoes is taken up, & the pressure spikes up.The valve, or
"Hold-Off" per Mopar terminology, set-point..or"Cut-In", is 117psi for std & tk., and 165psi for PoPo/Taxi apps.
Proportioning valves are there to balance front & rear braking forces by incrementally restricting flow/pressure to the rear as pedal/MC pressure increases, for a given amount
of frt. psi, the rears are reduced a certain amt., say 1000psi/540-650psi for a 3.2slope, & 1000psi/430-550psi for a 3.7slope.
4-wheel drums were basically proportioned by the wheel cylinder sizes, large in the frt. & smaller in the rear. Since they have identical operating properties, they functioned
OK in that manner. 4-wheel discs are also the same, caliper bore/total areas are proportioned the same to provide a correct balance, but both systems work better with a
proportioning valve because of the dynamics of hard braking. The weight distribution and available traction frt./rear changes dramatically under hard braking, so a system
has to adapt to provide optimum performance under all conditions. An adjustable proportioning valve is always a good idea, because You can fine-tune Your particular car.
