Loss of water pressure

Inspected a home last week with water pressure at the main line of 50 PSI. Plumbing in the kitchen with good pressure. When I went to the bathrooms the sink faucets had good pressure. When I ran the tub shower faucets had good pressure for about 30 seconds and then pressure dropped of significantly, cound not even get the diverter to operate and allow water to flow to the shower head. Any Ideas as to what would cause this. Thanks,

I have seen the cheap tub/shower valves fail like that in 2 yr old model homes that were unused for long periods.

Could be more serious but I doubt it is anything more than a bad valve.

Just what Bruce said. I recently had to replace a kitchen faucet that was doing exactly what you are describing, The faucet was only 4 months old. Come to find out it was trash in the line from the factory that would get free and clog the tiny orafices and holes in the plastic insert part of the faucet. Almost microscopic pieces of plastic and rubber from gaskets, etc. If it is restricted to the tub sets, I suspect faulty valves and possible trash. If a home sits empty for very long the O rings dry out and can crumble or pieces break off when it starts being manipulated again. Just did a big old house and every ball valve in the house leaked and there were a bunch of them. Seals and gaskets dry out and most often have to be replaced. One of the reasons people winterize a home.

On rare occasions I’ve seen the thermal limiter on the diverter do this. But that is on high end valves. If it is a three valve unit it is possible also, but that usually effects either the spout or the shower head.
New valve is quick fix.