Minigraf 104e hissing

[AndyP1]

I have recently bought a (well) used Minigraf 104e 2 position underpinner. It seems to work pretty well - I've used it a bit and all seems good except that there is a constant hissing sound which I am wondering about. I've never owned/used a pneumatic underpinner before and I'm not sure if constant low level sound of air escaping is to be expected or not?
Any advice appreciated - thanks.

[Not your average framer]

Continuous hissing means escaping compressed air. It should not be doing that. Can you figure out where the air is escaping from? Hopefully it's something you can easily fix yourself, otherwise you need to get someone competent to look at it for you and figure out, what needs fixing. Unfortunately, it is not possible to say what needs doing with so little information at this stage.

[AndyP1]

Thanks for replying.
I discovered that air escape is from a control valve unit - managed to find well-priced unused replacement on Ebay.
Thanks again.