Hi,
I am working on a Speakman Sentinel valve that has low cold water flow. When the valve is full hot, water sprays to the end of the tub, when turned down to a cooler mix (about 1 revolution) the spray only goes to half tub and at full cold the spray only goes out a foot or so. The valve is permanently installed behind tile. I have removed the ring of screws and taken the cover off the housing. The inside looks very clean and there is no crusty corrosion and no obstructions in the ports as far as I can see. The housing is dark brass colored.
Looking at the parts diagrams I have found on line, there is a center "piston" that is behind a threaded cap. I have removed the cap to uncover this assembly and there is tube extending from the "piston" body and I can see a couple of small bleeder holes. There is a film of green corrosion on the "piston". I have carefully grabbed the tube with visegrips and it will not move or rotate. It appears to be all the way back in the housing. with the tube extending out about 3/4".
I have loosened the three internal screws on the housing and can't remove the inner valve body either.
Is this piston supposed to move freely?
Is there any fastener holding it in place?
Do you think that this is the cause of low cold water flow?
It would be a major problem to replace the valve in the wall.
Any suggestions on how to proceed...
Thanks for your help.

Dave