Problem with my CNC drive, just wondering if anyone has experienced this and what was wrong.

When I connect the hose/chuck to the valve and push the plunger down I just get a hissing sound from the base of the pump as though the air is going straight out from there. Oddly (or perhaps not) the air runs through the chuck normally when not connected to a valve.

Went suddenly on Sunday so guessing it’s probably an O-ring or something?

Happy to try servicing it myself if that’s all it takes, just means having to buy a pin spanner to get the top cap off.