Grigg, AFAIK the residual valves are there purely to stop fluid drain back due to the M/C being lower than the calipers/brake cylinders on under floor applications,so would a firewall M/C even need them??
Even with the MC higher than the wheel cylinders supposedly residual valves provide for a firmer pedal feel on drum brakes, or rear drums if you have front disc. Because the brake shoe return springs have a lot more force than the wheel cylinder at 10 psi they do retract and don't drag, so no harm that I can see.
I think some proportioning valves "combination valves" have the 10 lb residual built in for rear drums..
Thinking about how they work I really am beginning to wonder if it makes any difference where they go, but like you, suspect that closest to the MC is still best.
Grigg
