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Is it doing this just when you first puck up the horn for the day or continously?

Because, if it's the former, then it sounds like there is just a difference in pressure between the open bugle and the valves' piping. I'm no expert or repair tech, but I often have this with my horns due to the 10-15 degree difference between the day and night for where they are kept.

Check that the bleed hole in the top and bottom of the piston are not blocked.

I understand what you are describing. I'm not exactly sure what causes it, but it is a serious issue, particularly in softer more sensitive passages. If I do not depress the valves, especially the 1st, prior to playing I get that pop.
I'm using a 2 year old BMB with very tight stainless valves. Extremely fast and smooth. I get the impression that the valves when being depressed are pumping or extracting a certain amount of air from the attending tubing, thus causing a negative or positive atmosphere and thus a pop. Venting the pistons should help, but that is one project to yet have accomplished. It seems to happen with all the valves, but the 1st is the most problematic due to the frequency of use. My goal: have them all vented.