I noticed someone got a couple of sensible replies to a question about air conditioning recently so I have a question of my own. The Aircon on our van has stopped working, it failed a vac test and a pressure test with nitrogen, I had a look and found a problem with the condenser so ordered a new one, took it back and apparently it is still leaking. Not getting any joy from anyone I bought my own manifold set and vac pump, only cheap ones but it should do the trick. The last place I took it to I got them to leave it charged up with nitrogen and checked it myself, on my gauges it stayed absolutely perfect over a few days at 90 psi and that was after a week from getting it charged up, didn't even deviate the thickness of the needle over the few days and that was with leaving the valves open on the gauge set and schrader valves. That tells me its not leaking at all. I then did the vacuum test, left it running for about an hour to get any moisture out of it, with the pump running it does pull a vacuum down to about 28 but as soon as you shut the valves off it leaks back to zero in about 30 seconds, how can it be airtight with nitrogen but have a bad vacuum leak? I don't get it at all.
Remember a vacuum is the same as having pressure in the other direction. So, maybe there is a seal somewhere which when under pressure is pushed hard against a surface and stops and leaks, whereas under vacuum, it is pulled off the sealing surface and lets air in.
Maybe, how do you find a vacuum leak though, if its under pressure you can squirt soapy water round everything and watch for it bubbling up where the leak is, that's how I found the leak on the condenser. With a vacuum leak you can't spray anything onto it as it will just get sucked inwards. I've got some iffy gas coming from Ebay in the next few days so will fill it with that and see how long it lasts, you can put a UV dye in with it and check with a UV torch for splattered liquid where its leaking. Maybe it will hold if its under pressure.
Their is a seal on the shaft of the compressor that is a carbon disc pressed against a hard metal running surface, this seals the shaft and the casing through which it passes. Having a spring behind it, it is designed to seal against pressure but may lift and leak on vacuum, this could be the problem, not a leak at all, but just a feature of the design of most vehicle air con systems.
maybe if someone is in Singapore but as a U.K. forum I doubt it’s worth driving there to fix the aircon on a van