The Residual Check Valve preloads the drum brakes with about 10 psi pressure. If you have front disk/rear drum, only the portion of the M/C for the drums has a residual valve. Disk brakes must not use a Residual Valve, but only need a weak pressure which naturally results from the M/C being mounted high on the firewall. However, some all-disc Master Cylinder use a low-pressue check valve, so check the application.
Sometimes called RLPV (residual line pressure valve), or RPCV (residual pressure check valve).
- Datsun 1200 all-drum and disk-drum dual master cylinders are identical, except the latter only has one residual valve fitted.
- When converting from all-drums to disk-drums, the master cylinder to match should be swapped. Or you could use the all-drum cylinder, but remove the Residual Valve from the "F" portion. The brakes will still work OK, but will wear faster than normal if the RV is left in place.
- Locking front brakes is not likely to be caused by a RV. More likely the M/C pushrod is not adjusted correctly, the Caliper pistons are sticking or some other problem. If using custom brake setups, the wrong combinations of parts may be fitted (e.g. proportions are off).
- If relocating a disk-brake master cylinder below the floor a residual valve should be fitted.
Older all-drum models use a single-cylinder master cylinder. These are unsuitable for use with a disk/drum combo. Plus not as safe, so you should convert to a tandem master cylinder, for which you need the cylinder and brake lines. These usually come with a differential pressure valve to light the dash lamp indicating a brake failure.
Late model Sunny Trucks use a disk-drum combo with a tandem master cylinder with SINGLE brake fluid reservoir. Although using a single reservoir, the cylinder is a dual unit, and has separate front and rear line outputs.
Brake proportioning is not a function of the Residual Valve. Datsun 1200 disk brakes require a higher line pressure [harder pedal push] to make them work & this means that the rears have a greater tendancy to lock up, so the factory fitted wheel cylinders with a smaller bore size to balance the brakes out. Thus a correct proportion is achieved, which is different from the all-drums proportion.
If you fit stock 1200 type disc brakes, then modify or change the master cylinder (remove the RV from the front circuit) AND replace the rear wheel cylinders with 11/16" disc brake units.
All Drums Front: 13/16" Rear: 13/16"
Disc/Drum Front: 1.894" caliper cylinder with 212.5 mm rotor Rear: 11/16"