In addition to all the causes mentioned so far, it may be that the tolerance between the chamber and the piston is too tight and, consequently, there is a problem of attachment. Look at the piston and see if there are any smooth areas on the surface, as if they were mirror polished which differs from the more opaque rest. If so, you can try to lightly sand the shiny surface with fine sandpaper or cloth (grain size 1200). I have done this operation in the past on various horns with success without compromising the seal.
Last edited by franz; 10-26-2022 at 09:51 AM.
2007 Besson Prestige 2052, 3D+ K&G mouthpiece; JP373 baritone, 4B modified K&G mouthpiece; Bach 42GO trombone, T4C K&G mouthpiece; 1973 Besson New Standard 3 compensated valves, 3D+ K&G modified mouthpiece; Wessex French C tuba, 3D+ K&G modified mouthpiece.