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Can a non-repair man check for leaks, my new-to-me Conn 12j feels like it has a leak with some of the valve combinations. Could just be my trombone chops doing a terrible job of finding center of the pitch. Anybody know how I can find the leaks or does it take some kind of compression tool? Thanks

by the by, what mouthpieces do you other Conn 5 or 12 players use for the best tone considering the instrument.

The 12J is a nice, simple tuba; a good choice for a trombonist.
The best way to check for leaks is to wait for a warm summer day, stick a garden hose into the mouthpiece while holding the tuba upright on the ground, and turn on the water. If it leaks you will see the water coming out from the trouble spot. (Remember to depress the valves so that water flows through the valve tubing.) If it does not leak then you will see the water level rising in the bell, along with whatever paper wads and green monsters may have been in there. You may see a few drops oozing out from the valves, but if it's a steady dribble then you have a problem there.
If you can't wait for warm weather you can try this: Pull out the main tuning slide, and stick its small side back in. Hold your thumb over the larger open end, or stick a cork in it. Blow into the mouthpiece receiver and listen for air leaking out. Leaks are harder to detect and locate this way, but you may be able to find the problem. Of course you won't get the paper wads out this way.
As for the mouthpiece, I would recommend a Kelly 18.

Make bubbles with water and dishwashing detergent (Dawn, Palmolive) , plug the end, have someone blow in the horn and start spreading the bubbles around. The person blowing does not have to put much pressure, the leak will be obvious.