Mine did exactly the same until it began to fail to compress air altogether. I assumed it was the piston seal but it turned out that it was the only seal that wasn't shot. Diagnosis and repair were carried out by Laceys Shooting Services- first class service by Chris.
I would add that I do not know which seal failed first as I switched over to another gun and left the 600 in a state of disrepair for some time. So its likely that the other seals may have followed the offending one/ones.