Water temperature mixer in shower

[LA1]

See picture below...as of late, when the temp dial is set on hot ( 100° ), half the volume of water comes out as when on cold. Until I discovered this I thought that we were suffering from low water pressure. We also have a hand held sprayer on the other wall that is affected too. The water faucets are fine..from hot to cold same pressure, tube in the washing machine fills ok.
For the shower the main control as you went from off to on to full there was always an increase, now when you get to about 1/2 way, after that it is the same pressure unless on cold. So what do you think, is this a DIY or a job for a plumber, how much should it cost.This was in a bathroom that had a tube, there is a panel in a closet that has access to the back only at faucet level for the tube, so possibly a hole will have to be knocked in the wall to service the valve...could it be just an adjustment for the hot watre control.
thanks

[hj]

There should be a pressure balancing spool of some kind inside the valve and it is sticking, or broken. Remove the large plate and see if there is a "nut" of some kind on the front of the valve, usually at the lower right, but maybe on the left. IF so, that has to be removed and a new one inserted. BUT, you have to purchase the one for that brand of shower valve. If there is not that piece, then the spool is probably integral with the control unit and you have to replace that part.

[LA1]

Not quite sure what exactly you mean about dissasembling. I thought that if I took the knobs off followed by the "tumbler" style temp dial I could remove the plate. I removed both of the knobs. The temp dialer seems to be integrated into the assembly with no visable way to remove the tumbler. Actually the wall plate seems to be part of the assembly instaed of going in back of the tumbler. I will look in the service access in the back tomorow. If the unit is one peice::
1. what would be a ball park price to replace (labor)
2 " " " (part)
3. what brand / model should I replace with ?

[LA1]

This is what I have...found this manual in a cabinet



...here is the exploded view



...here are the instructions



In the instructions it says to turn off the check valves. can they be accessed from the back..what does a check valve do and what are the symptoms of a bad check valve?