Common problem there is a chance you may have to rethread the threads to screw the seat in but before you go to that great length try putting a little teflon tape around the seat and a little pipe dope and tighten it up snug get the real wrench set so you can make sure you are tightening it up enough this has saved me a good 75% of the time on these but sometimes its so worn that the seat starts to slip and will need re-threaded or valve replaced but give that a shot let me know if it helps hopefully it takes care of BTW some of these valves have conversion kits that you can actually put in seats and stems that turn it into a cartridge style quarter turn instead of the old compression just a heads up.

Ben Franklin Plumbing Of Alpine
304 Meadowlark Drive, Alpine, UT 84004
(801) 960-1567 ‎