Tech information furnished by Brian Swearingen ~ Eric Stegall ~ Morgan Fletcher
The residual pressure valves are located in the master cylinder where the brake lines attach. These 10# valves are designed to maintain minimal brake pressure drum brakes. With disk brakes, the calipers don't have enough force to push back the 10# of pressure since there are no springs to pull the pads apart like on drum brakes.
If running a 76+ disc/drum combination FJ40 master cylinder (shown), pull the flanged line nuts/fittings off for the rear braking system. If running a 75-older master cylinder, pull both. Remove the large nut for each, where the lines connect in. (IIRC, 15/16" or so) Inside there you should find a white piece of plastic, relatively round, with a tiny spring on the bottom and 4 tiny holes in it. (See photo) This is the residual valve. Remove both, toss over right shoulder for luck, and replace all connections. Re-bleed system.
You have now successfully removed the residual valve from your system. A 71 drum brake FJ40 master cylinder should have this valve in both fittings....a 76+ drum/disc combo would only have one for the rear drum brakes.