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Just from the pressure of playing, not slide adjustment, my valves "pop". Most especially the 1st. Not just from the slide part, which I can artificially clear with the water key, but also from what I can only think must be the Blaikley comp loop too? Is this a thing? The valves seem to have very good compression with the exception of the 2nd which is not as good as the other 2 but still fine. It's most noticeable in a small practice room, but it does effect the air flow making certain passages more tricky to play. Has anyone ever vented the comp loops? The horn is not vented at all at the moment.

Any thoughts most welcome as I'm slightly puzzled as to how the pressure is getting in there! Am I blowing to hard?

No idea what your problem is, but...
When you depress a piston valve it must vent trapped air.
This is done on a Besson through the interior of the valve, itself, and out through the hole in the bottom valve (casing) cap.
If there is a blockage to venting it may pop, but in your case....?
Anyway, remove the valve and remove the button and top valve cap.
There is a tiny hole in the top of the valve which is the vent port - this can sometimes get blocked.
So trapped air vents through this hole, through the centre of the valve, then out through the bottom of the valve.
Make sure this is clear - flush with clean/ warm water.
Then check the the bottom cap hole is not blocked or that any rubber spring dampener used in not blocking it, either.
The above is what I would try.

Geoff, the valves aren't vented so there is no connection to the centre of the valve. So even if these holes were blocked it would make no difference!

I think I've worked out what's going on. Boyle's law. I've been storing my horn at 13C and playing it at around 15C in my "music room". Blowing warm air into a cold (and not really fully warming up) tuba. So the air in the slides is cooling rapidly and causing a pressure change. This is released on depressing the valve (Phut!), but the air continues to cool and it happens again on the second and subsequent presses until it gradually fades.

Cold weather carolling (a UK Brass Band staple) could benefit from venting.

As an aside my valves literally froze solid on Thursday playing out at -3C. It took constant blowing through the horn to keep it above freezing to get them going again, and between numbers to stop the freeze.

I just found the same effect after taking my Tenorhorn from the cold garage to the house. It took some time of warming up the whole instrument before the effect disappeared.

Intention to vent is go...
3d Printed Jig made and now for the bum squeaker....

https://www.dropbox.com/scl/fi/v9s9m2hq ... 0wxri&dl=0

Done, and easier than I thought it would be. If anyone is interested I'll post up some photos of the process.

Sure, owning the predecessor model, I am quiet interested how and where you did that. Thanks!

A photo sequence...

https://www.dropbox.com/scl/fo/bh7n29pg ... 45ryi&dl=0

Thanks a lot!