Air Con suction port

[Waynester2]

Hi Guys,
Trying to top up the air con on my D2, as the compressor is a bit noisy, and in the past with VAG cars I have found this could be due to low gas.

Basically I have found 2 ports, one on the front which is fitted to the condenser (discharge) and one in the middle just below the windscreen, which appears to be suction. To check the pressure I need to attach to the suction port but it doesn't appear to be a standard automotive fitting, anyone know how to attach to this?

Otherwise I'll have to vac and weigh it in, via the condenser port and I'd rather check and top it up.

Thanks!

Wayne