Residual Valve?

I can't seem to find it, but I remember 7-12PSI being the amt. the valve would hold in the line to keep the wheel cyl. cups tight against the bore, preventing air/moisture
from entering on cool-down. I guess a Teed-in gauge reading much of anything over -0- would say yes. The '74 FSM stated that tandem M/Cyls. did not use residual check
valves because they switched to the spring-loaded cup supports in the wheel cyls. The only M/cyl w/one was 1ton tks/vans, anyone with more intimate knowledge can jump
in and correct me if this is not correct. I'm sure the aluminum late model RWD units are sans residual check valves.