unfortunately you’re having to strip it down.
you need to test the seal on the nozzle <> T piece.
the vector’s stock t and nozzle have a fairly unique profile and its been noted a few times that many of the after market t pieces leak like a sieve because of poor contact with the nozzle rubber / silicon.
now, why you don’t experience this in burst / full auto is easy…simply…the box keeps hammering away at the cylinder shoving air into the system so what you see at the other end looks ok.
once you fix the seal, your full auto and burst will be off the chain by comparison to what you presently see as “ok”
as will your single shot.
checking the seal in the vector is a bit of fkn around…but the way i do it is…
stuff bluetack into the gel input hole
make sure your piston is seated in the cylinder and that the nozzle is all the way forward
blow down the barrel…
if you are leaking you’ll hear it
if you have a good seal you will feel resistance and if you blow hard either the piston will move (if your gearing isn’t installed) or you’ll blow out the bluetack…or go blue in the face trying.