A gate valve is a low-tech solution for controlling the flow of fluids within pipes. It has been used for more than 100 years. A gate valve works by raising and lowering a gate into a seat with a screw mechanism to control the flow. One of the weaknesses of the gate valve is that is shouldn't be left in a partially open position. If left like this, the closure element will become worn, thus losing its seating integrity. Repairing a gate valve is possible with some specialised tools.
Shut off the water before the gate valve by closing a valve upstream of it. You may have to close the main water valve to the home if there is not another option. If it is your main water valve that needs repairing, you will need to contact the city to turn off the water at the street. Open a valve downstream of the gate valve to remove the pressure from the pipes.
Remove the turning mechanism from the gate valve with the appropriate tools. Most turning mechanisms will be secured by a single slotted screw in the centre of the handle. Loosen this screw to remove the turning mechanism.
Remove the retaining nut from the top of the gate and then remove the gate from the valve by turning it counter-clockwise. Examine the gates stem washer for evidence of damage. If the stem washer is damaged, replace it with a new part from the hardware store.
Remove the gate valves seat with a seat removal tool. The seat removal tool maybe specific to your faucet. Examine the valves seat for evidence of damage. If the seat is not smooth or uniform replace it with a new seat.
Reassemble the faucet the same way you to took it apart. Test it by turning on the water supply.
Not all gate valves can be repaired. You may have to replace the entire valve to correct the issue.