Low pressure shower

[John45]

Our tub has a Delta valve for shower head and tub faucet. It's 13 years old. Suddenly one morning we noticed the pressure dropped by about half and when we shut off water after shower almost no water leaves the faucet. We cleaned the shower head but no difference. Replaced shower head no difference. Called plumber he replaced cartridge but no difference. Removed flow restrictor and filter from the shower head. That helped some. The faucet seems to flow normally when filling the tub. So now we have about 70% flow which we can live with but interested in learning what might be wrong. Our plumber suggested replacing the valve but that's a big job.
My guess is that somehow the valve feeding the pipe up to the shower head has become partly plugged. Reduced flow goes up to the shower head but acts like a check valve when water is turned off.
Very interested in what a plumber might know and if there is an easier fix.
Thanks

 

[Sylvan]

Shut the water supply, disassemble the shower body, and check to see if any debris is blocking the valve
Check the cartage carefully

 

[Sponsor]

 

[Reach4]

If you remove the showerhead, and run water, maybe the flow will wash out the debris.

 

[Jeff H Young]

good guesses that something is plugged these problems can be costlyn unless you have a free or cheap plumber . I determined my parents house had a plugged valve on a old delta the type with the ball no pressure ballance the valve has a small passage way internal that goes up to the shower i tried foir a long time to clear it and gave up and settled for some improovement . tried compressed air soaking it with clr . woulda been faster to replace the valve but I was outta state and limited tools house was being sold it was ok but definately had a blockage .
With a modern pressure balancing if either the hot or cold line is partially plugged up it effects both hot and cold