Hi Guys, I'm new here and have tried a search but can't find anything that properly answers my questions so sorry if this has been asked already

I recently picked up a 2008 Mk1 Mazda 3 and having a bit of trouble with the Air con.

It wasn't switching on so took it to a garage who said it may just need re-gassing but would need to be booked in for another time. So I went and got a re-gassing kit from halfords as I've done it myself on other cars in the past.

Plugged into the Low Pressure valve (with engine running and air con on) and couldn't get a pressure reading. So Assuming it was empty I put some gas in, got the pressure reading up to around 35psi and heard the Compressor / radiator fan kick in and my pressure reading disappeared. When I took the pressure gauge off I heard hissing from the valve, so the valve was leaking, I have changed the valve core using a valve core extractor and it no longer leaks.

Now though I still don't get any cool air, the moment the air con switches on I lose my pressure reading.

Questoin I'm asking is if anyone knows what the pressure readings should be for the air con system? When it is off, I get a reading of 35psi, when it's on the max reading I can get is 15psi.

any suggestions are appreciated