Piston valve problem.
I should start off by saying that I am certainly not an expert in this area and there are several members of this forum that repair instruments for a living that routinely give GREAT advice and might give you different advice from what I'm about to suggest (which is one of the reasons I love this forum).
Having said that... I think one thing you could try is to switch to a different valve oil. I was having a similar problem with my euphonium and my baritone, both Yorks with stainless steel valves. I was using Al Cass valve oil and I switched to using La Tromba. I believe the difference here is that La Tromba is synthetic and Al Cass is not. Since the switch about 3 months ago, I have not experienced the same problems. There are other oils that people swear by as well that could also work... Yamaha synthetic, Hetmans, etc.
If you try this, make sure to clean out the casings again (don't mix the different kinds of oil). Good luck and welcome to the forum!! I hope you get as much out of it as I do!
Mike Warner
Puget Brass
York Preference Baritone 3055
York Eminence Euphonium 4052