Re: little shower hot water pressure
My suspicion is that there is something plugging the water inlet to the valve, most likely rust deposits from old galvie pipes. Be forewarned, turning off valves along a rusty pipe will break these rust deposits free and clog other fixtures, though usually it's the aerators that clog, so before you run any water anywhere in the house after messing with a valve, remove the aerators and shower heads from the fixtures. Run the water until you're certain all deposits have cleared, then reinstall the aerators.
What you may have to do with the shower valve is disassemble the valve to clear the blockage. If you are not up to that task, I recommend hiring a plumber to clear your lines for you.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!