Planing help.

[Gurp]

Ahhh that time again... My least favorite thing to do other than fold laundry in the house.....
Plumbing...
So my issue is when I turn my shower on I loose about half my water pressure out my tub faucet. If this a rebuild able vavle like a sink or do I need to replace the the whole setup?
Really hoping to just fix this quickly rather than redo my shower mid winter.

 

[Half-fast eddie]

Got a picture of the tub spout? Is it the old style with the lift-up dealie on the spout, ir opis the selector on the valve body?

 

[Gurp]

It's got the selector know in the center.