I have a problem with a two handle faucet on a stall shower. Recently neither the hot nor cold water turns on immediately when the handle is opened. There is about a 20 second delay. During that time some small amount of water trickles out and then the water flow suddenly becomes normal. This happens on both the hot and cold sides. The only change I have recently made was to install a washer on the hot side about 4 months ago to correct a drip from the shower head. I removed the valve stem and installed a new domed washer. That fixed the drip. Thereafter the shower worked fine. Now several months later I have this problem with the delay when the handles are turned on. At first the delay was not long, but it has gotten longer over the last month.

It seems to me that after the shower is turned off the water must be leaking out of the line between the shower head and the valve. However, there is no evidence of leakage in the ceiling of the first floor below the shower. I am on a municipal water supply and have excellent pressure everywhere else in the house. I have stopped using the shower until I can figure out what is happening.

The house was built in 1974 and has the original valve and stems.