Check the listing for details. 1978 1987 Chevy GMC Full Size Truck Black Brake Proportioning Valve Combo Disc. Condition: New. Listed at 33.88 USD. This is a Brand New Black Painted 1978-1987 Chevy GMC Full Size Truck Brake Disc/Drum Combination Proportioning Valve This valve is a direct replacement valve with no modifications needed. No need to cut any wires with this valve even has the correct switch! This Works on ALL 1978-1987 Chevy & GMC Full Size trucks that have disc brakes in the front and drum brakes in rear. 2WD & 4WD, 1/2 Ton, 3/4 Ton, & 1 Ton! Even the P30 Chassis Frames. This brake valve is a 5 port valve and will work as left mount, right mount or bottom mount. The valve has 9/16"-18 connection for Rear Brakes & Dual 3/8"-24 ports for the front brakes. The connections from the Master Cylinder are 1/2"-20 and 7/16"-24. The Proportioning Valve allows proper fluid transfer and contains a residual pressure valve for proper braking.