My Symmons Temptrol (Model B) temperature balanced shower valve is only
putting out cold water. Symmons lists this common problem on their
website, http://www.symmons.com/service/product_info.cfm#Q6
and first fix is to remove the handle and tap the spindle a few times
with a rubber hammer. That didn't work.

Next, the solution is to remove the spindle assembly.
http://www.symmons.com/ins/temptrol_ins.pdf#page=5 (pdf file)

Here's where I'm having trouble. I can't remove the cap (part T12-A).
I've taken a wrench to it and it won't budge despite a lot of torque.
I'm afraid to apply any more force on the wrench because the valve is
supported by the plumbing inside the wall.

Has anyone had this problem or know what to do? Perhaps heating it a
little with a torch would expand the cap enough to unscrew. Thanks!