you might want to pull the ball and washers out of the faucet and then
turn on the water to the home for about
10-15 seconds or more....
Close the curtain to the shower room and let it blast out to see if you blow
some debris out of the faucet....
of course if you have ball valves in the lines you can just power flush
just the hot side if you so desire...
I have blasted out a ton of shit before that I would have never known was
lurking in the lines or body of the faucet
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Back-flushing
I have also had to " back flush" older systems before to blow everything back into the water heater
this takes a lot more prep to do......
basically you have to capp off the shower head and capp off the tub spout so water cannot
escape either way....
then you turn off the water to the heater and you drain off about 5 gallons off the bottom of the
heater down the drain.....
then you turn on the tub faucet to the middle setting so the cold water blasts back down through the hot side of the faucet all the way back into the water heater.....
if anything is stuck in the line,,,, in theory it will simply flush back down into the bottom of the heater
where it will lay forever.....
This process has gotten me out of a lot of troubles in older homes with galvanized lines over the years
and made the hot side of the faucet more acceptable ....
you might have to turn it on and off a number of times to dislodge things in the line
then just put it all back together again , turn on the water heater
and hope that it solved the problem
good luck