Might I be side loading the valves? Oh yes that's very possible. Broke my right wrist twice in the last two winters and kept playing while it was in a cast. My tech also thought it might be the ultra fine nylon bristles of a HM brass saver brush had possibly lodged between the pistons and their casings.
Does the valve leak definitely. Plug the horn and blow first slide starts to extend itself.
Can't help but wonder if the horn is worth the some of its parts or if I'd be better served with a better horn.