shower, and disassemble the valve, them flush it out by briefly turning the
water back on, with appropriate precautions.

if you try this dont use the main houses valve, i did once it wouldnt
shut off and flooded the basement by the time the water company got
things turned off.......

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You could test it first.

a old valve that works fine can fail under a high unlimited flow
situation, i learned this the hard way