How do you know it's not seating? If your headers are dry at idle it is seating. If not then I'd order a ball and spring and drop them in to do a cheap test. Perhaps the water pressure at idle is opening up the spring a little. Gotta troubleshoot the system.
Hold the bus > before ordering new parts > sometimes debris can get between the ball and its seat...happened to me a few times before I started filtering the river/lake water into the T.
Also, I keep my T mounted to where I can get to it 'if' I suspect a problem > simply remove the inlet hose and poke the ball with a screwdriver etc. It's also good practice to spray some WD40 into the checkball area.
Also, you can simply put the T in a vice and unscrew top to visually check the ball, spring and seat for potential problems.
For some 15 years the Bassette T has been a trouble free system > especially AFTER I added a pre-filter into it.
i took it off and cleaned it then i blew through it . and it doesnt seal tight .
the ball and spring look good .
i have two valves and put the good parts into both with the same result
Ball chatter may have hammered the seat...try taking one ball, set in in the T in a vice and with a brass drift give it a decent solid but not killer hit. Then, toss that ball and insert the other ball and check it.
You could also weld the ball to a rod and use lapping compound to dress the seat up > a new ball will still be needed.
Or, just order a new T
BTW you can shim more 'closing pressure' on the spring with a stainless washer or two.