I'm no expert at this but I think most systems use residual valves to keep
the fluid from bleeding back into the m/c and that for disc brakes you need
one with a higher psi rating. The residual pressure keeps the caliper
pistons out so you don't have to pump the brakes (because there is no
adjuster on a caliper). I believe that they are built into the junction
blocks on most factory set-ups. In regards to m/c's that are mounted high
vs low, if the m/c is mounted below the wheel cylinders or calipers than I
think gravity comes more into play as far as the fluid bleeding back into
the m/c.
Proportioning valves are needed if you have a brake system that is not
specifically designed for your vehicle. Pick-ups with-out anything in the
back tend to be light on the back wheels, so you need less brake at the rear
wheels to keep them from locking-up.
Hope this is of some help.
Joe Szwed
50 chevy 3/4 ton
51 gmc 1/2 ton
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