[lurkingknight]

mismatching brands of hopup unit to gearbox shell and nozzle has a potential to cause this. This is why you try to not change any of the 3. The nozzle is usually easiest because you can generally find one the exact same length as stock or just a bit longer to shave down.

You make this harder by changing the hopup unit as the feed tube may not sit proportionally the same location from one unit to another or the barrel is held more forward than backwards or the hop unit itself sits farther forward or closer to the face of the gearbox, or the gearbox shell is thicker or thinner at the front or the tappet channel is not as forward, etc etc.

Changing shit around so now you have to play with trying to put longer nozzles in to reach the lips of the bucking can cause feeding issues causing you to sand the nozzle tip which can then lead back to air seal issues. Some CA units are even too narrow for the nozzle to protrude all the way into touch the hopup rubber.

And assuming the dimensions are all the same the hopup unit could be cast too tall or too short or at a slant that causes a nozzle to hopup misalignment causing the seal to break.