Hy PhilBurban, for years and years drum brake master cylinders have had resisdual pressure valves built into them, they are there to keep the drum brake wheel cylinders "charged", so that no part of the pushrod stroke is waisted "recharging" the wheel cylinder. I've said it before and I'll say it again, I'm not an engineer, and I won't modify a proven and safe design. A GM engineer once said "parts left out cost nothing, and cause no service problems, so you can be sure if we put it in there we feel it is necessary." Resisdual pressure valves have been in every drum brake master cylinder I have ever worked on, regardless of the mounting location on the vehicle, they are also used in dual systems with disc/drum configurations, and 2 p.s.i. resisdual pressure valves are specified in disc/disc systems where the master cylinder is mounted below the level of the brake calipers.
