Compressed air to confirm leak

[Rogerson]

Hello fellas
Just a general question on leak testing
Can compressed air be blown up the evacuation port of a manifold set with the low gauge tap opened briefly just as a means to trap the air and find a leak in an empty system, will 70 psi will be sufficient to test for a leak. Heard it can put moisture in the system and shouldn’t be done, wudv thought pulling a vacuum sucks out moisture as part of a shop regas. I don’t really have much else at home to use. Iv even heard of it being done to start the vehicle for a few secs to check the compressor cycles
In Australia you have to have a refrigerant licence to buy r134a gas. Can’t buy cans over the counter. I’d be wasting $250 getting it regased if there’s still a problem. I thought I’d ask the AC specialists in here wat I can and can’t do. I thought it be a better way of confirming leaks rather than looking for refrigerant tell tale signs UV light etc

[DetroitAC]

I can see the spot you are in, difficult to do things when you are not allowed to do things!

I'll admit to having used compressed air a few times when in a tough bind. In general it's a sin and a last resort, the reason is that compressed air by it's nature contains a lot of water, and water in refrigeration systems is really bad, there is dessicant to absorb whatever sneaks in while servicing and whatever permeates in through the hoses. When I've had no other choice but to use air, I've followed it up by a long hard vacuum and a new dessicant bag, also an oil free supply of compressed air. In my case I had a plastic piston air compressor at the time and blue dessicant beads downstream of the air compressor.

Before y'all roast me for giving this guy this advice, please put yourself in the OP's shoes.

In you situation I would buy or borrow an oil free compressor, buy or borrow a dessicant drier, cheap one like hobby guys use for airbrushing, find your leak with squirting window cleaner on every fitting, joint, etc. and then when you have it fixed install a new drier and take it to the AC shop for evac and charging.

OK, I've got my cast iron underpants on, come at me

[JohnHere]

Hello fellas Just a general question on leak testing

Welcome to our USA-based Forum.

Before y'all roast me for giving this guy this advice, please put yourself in the OP's shoes.

... OK, I've got my cast iron underpants on, come at me

Not at all. In fact, your reply covered just about everything I was about to say...and then some.

The only additional concern that comes to mind is the oil. If you pump in damp shop air, there's a risk of contaminating the PAG oil as well because it's hygroscopic (that is, readily absorbs moisture), unless the system contains double-end-capped PAG, which is less hygroscopic. Chances are it has regular PAG in it, though.

I believe that if the OP follows your advice about the compressor, desiccant dryer, etcetera, the risk of contaminating the oil and the entire system will be minimized. I don't think it can get any better than that under the circumstances.