About to plumb in the residual pressure valves in my brake system when it dawned on me that I thought I heard something about they had to be below the master cylinder? My mc is mounted under the ch***is. I'm sure it because of possible trapped air bubbles, but that shouldn't be hard to eventually bleed out, right?
Residual pressure valves retain a minimum brake line pressure to help eliminate excessive pedal travel in both disc and drum brake systems. The two pound valve is used in disc brake applications where the master cylinder is mounted below the horizontal plane of the calipers and fluid drain back occurs from gravity and vibration, thereby causing excessive caliper piston retraction and a longer brake pedal stroke. The minimal two pound residual pressure prevents fluid from flowing back without causing the brakes to drag.
The residual valves are no more than a spring loaded check valve, when the fluid returns to the master cylinder as fluid pressure on the brake side reaches the pressure slightly under the spring pressure which is on the master side... it closes. The spring makes the system think it has equalized when it is actually holding a higher prescribed pressure on the drum/disc side. In a perfect world they should be mounted as per manufacturer's recommendations, but we don't live there. Once the air is out... it is out.. in a properly sealed and operating system.
Frank, The answer to your question is the residuals should be located as close to the master outlet ports as possible. JFYI, the main purpose of residual valves was/is to prevent air from being pulled past the wheel cylinder cups during fast brake release because of the strong drum brake return springs. They also help "tighten" the system and can reduce pedal travel. The 10 lb valves should be used on all drum systems (if not internally in the master), regardless of master location, even though cup expanders made the residuals obsolete by the early 70s. The 2 lb valves for disc brakes should only be used if the master is floor mounted, to, as has been noted, prevent fluid drain back. These were never used on production vehicles with disc brakes, because there were never any US vehicles with floor mounted masters and spot disc brakes. Some of the very rare disc options from Chrysler and others with floor mounted masters in the 50s had return springs because these designs were much like clutches, and actually used wheel cylinders for actuation, whereas modern spot discs rely on seal retraction to pull the piston back into the caliper, and anything higher than 2 lbs can cause the disc brake to "run away" and fail.
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