The majority of under-sink water shut-off valves, also called "angle stop" valves, fall into the general category of compression valves. When you turn the valve's handle to the "off" position, a rubber washer compresses against the valve's interior inlet to halt the water flow. Parts like the rubber washer tend to deteriorate and fail, making under-sink water-valve replacement a common plumbing task. If you're new to plumbing, repairing an under-sink valve provides a great way to learn the basics and, if you have some experience, it'll be a breeze.
Shut off the water supply to the shut-off valve. Place a bucket beneath the valve. Attach an adjustable wrench to the supply-side nut of the flexible water supply line that connects the shut-off valve to the sink. Twist the wrench counterclockwise to the loosen the supply line. Unscrew the supply line from the valve by hand and allow water to drain into the bucket. If an escutcheon surrounds the pipe that supplies the valve, use a screwdriver to remove the escutcheon.
Place a pipe wrench's jaw around the pipe that supplies the valve, and tighten the pipe wrench's jaw to grip the supply pipe. Attach an adjustable wrench to the hexagonal collar of the shut-off valve. Hold the pipe wrench stationary with one hand and twist the adjustable wrench counterclockwise with the other hand. If the valve does not budge, spray penetrating lubricant into the joint between the valve and the pipe, allow the lubricant to work for a minute and try again. Finish removing the valve by hand.
Brush the threads of the supply pipe with a stiff brush to loosen old thread sealant and corrosion. Wipe the threads of the supply pipe with a rag to remove debris. Unravel a few inches of pipe thread seal tape from the roll or unscrew the cap of the pipe dope canister. Wrap tape clockwise around the external threads of the supply pipe or brush a generous layer of pipe dope around the threads with the brush attached to the underside of the pipe dope's cap. If the supply pipe has internal threads, apply the tape or pipe dope to the external threads of the new valve.
Press the threads of the new valve against the threads of the supply pipe. Twist the supply valve clockwise to connect the threads. Turn the valve clockwise until handtight. Attach a pipe wrench to the supply pipe and an adjustable wrench to the hexagonal collar of the new valve. While holding the pipe wrench in place, turn the adjustable wrench clockwise to completely tighten the valve onto the pipe.
Place the escutcheon over the pipe and reattach with a screwdriver. Press the flexible supply lines threads onto the new valve's threads and twist clockwise to engage. Turn the flexible supply line's nut clockwise until handtight. Finish tightening the supply line with an adjustable wrench.
Turn the new valve's handle to the "off" position. Turn on the water supply to the valve, return to the valve and check for leaks.
Open a faucet in another room to encourage the pipes to drain faster.
Do not apply thread seal tape or pipe dope to the connection between the flexible supply line and the shut off valve; this connection seals with an internal rubber washer.