I am having drum brakes in all corners, and 1989 Dodge van master cylinder under the floor, probably lower than wheel cylinders. I know that it is recommended that one should have 2PSI residual valves for front and rear brakes in this kind of setup. Could this Dodge master cyl. have built in residual valve in front or rear circuit? How to make sure? Main question: How was it dealt in all those prewar cars that have master under the floor? Were these running without any residual valves?
Those old single piston master cylinders that were stock under the floor had residual valves installed in them.
As stated, all older drum masters had internal residual valves up to the mid '70s. Your '89 van master is a disc/drum and may(?) have an internal residual in the rear brake outlet port, so if present just add a 10 lb. external valve to the fronts. If one is not found, install 10 lb. external residuals to each axle. The 2 lb. valve is ONLY used for disc brakes, and ONLY when the master is below floor mounted.
Never seen a n early master with built-in residual, this is why they were constantly adjusting brakes up all the time as to keep the pedal up. As far as drum brakes all around there should be a 10 lbs residual front and rear other wise you will have a low pedal. This only for a master below wheel cylinder. Masters higher don't necessarily need them as the pressure from gravity will take care of it . What you will need is a proportioning valve
If you ever disassembled an early master, or looked at an exploded view, you would have found the residual. Residuals are necessary for drum brakes up to around the mid '70s, regardless of master cylinder location, but are also a good addition with later drum brakes. And no, you don't need a prop valve with a normal drum/drum system. These were only used for production disc/drum and disc/disc systems. An exception could be early non-servo inefficient front brakes with modern more efficient duo-servos in the rear, the complete opposite of how a well designed brakes system should be.
The residual valve in older master cylinders is part of the M/C piston assembly, not an auxiliary part like the add on type usually discussed here. The residual valve had nothing to do with the need to adjust the brakes from time to time. That was a natural consequence of brake lining wear. Self adjusting brakes (when working properly) took care of the need to adjust periodically. Ray
1942 Chevy manual - item 12 - return valve http://chevy.oldcarmanualproject.com/shop/1942_47/4247csm501.htm "The primary cup is held against the piston by the piston return spring which also retains the return valve against its seat. The spring maintains a slight pressure in the lines and in the wheel cylinders to prevent the possible entrance of air into the system. " The lip type of seal used in wheel cylinders and MCs are moderately famous for sometimes causing air entrainment issues in disk brakes that used them ( C2 and C3 Corvettes)
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