Uneven water flow, low pressure when the pressure at other faucets seems all right, or troublesome switching back and forth from spray head to sink spout can be caused by a malfunctioning diverter valve or by a restricted hose. To check the diverter valve:
Step 1: Remove spray head at coupling, and disconnect coupling from hose by prying off snap-ring retainer.
Step 2: Turn on water and let strong stream of water flow into hose. If strong stream of water flows out of open end of hose, then you know diverter valve is the source of the trouble. A weak stream flowing from open end of hose may indicate blockage in hose itself. Briefly running water full force may clear hose.
Step 3: If above steps don't locate problem, remove hose from spout attachment, stretch it out straight, and look through it while aiming it toward strong light source. If hose appears to be clear, problem lies in diverter valve. If hose is blocked, clear it with wire coat hanger or length of wire.
Step 4: As needed, replace hose. If you can't get exact replacement, adapters are available for connecting other types and sizes.