Hello.
I did it like 6 years ago and went well. Tried today and nope.
As far as I know, f there is excessive pressure, compressor clutch toes not engage. That is the normal 'cycling' behavior. If there is too little pressure, it does not engage, being the case now on about 24psi.

A new can of R134a, hooked to the low pressure port, does not fill as the compressor does not engage in idle due to not enough pressure. What am I doing wrong ? How to force the compressor to suck the gas ?

 

It will fill, just takes longer with the compressor not running. But you should be able to jump the compressor relay to get the clutch to engage I would think.

 

If the system is empty or very low, it can indeed take a bit for enough static pressure to build before the clutch opens. Just give it some time. The full system capacity is 17 oz IIRC and you may have to start a second can if it was completely flat.

It's also possible your leak is worse and you will never build enough pressure to start the compressor. Might be time to get the manifold hooked up and validate pressures.