Technical AC system won't hold vaccuum, but holds pressure?

I just plumbed my AC system and the vaccuum test bleeds down within minutes...or even moments.
So I figure massive leak. That being said I pressurized the system with shop air to 80 psi for a soapy water bubble test but it doesn't leak under pressure, but leaks like crazy under vaccuum. It will hold 80 psi overnight but can't hold 30hg vaccum for 30 seconds.
What am I missing?
Ps: My gauge set will hold vaccuum when I check it not hooked up, so that should eliminate an equipment issue.

 

Interesting, I've seen systems that will hold a vacuum but leak under pressure. I suppose it is possible that you have a schrader valve leaking when put under a vacuum. You might try putting caps on any schrader valves when vacuuming. Also, using shop air to pressure test isn't a good idea. You are introducing moisture to your receiver dryer. Refrigerant or dry nitrogen is a better choice for leak testing.

 

I don't know the specifics of your system, but it's entirely possible that some of the seals that are designed to hold pressure are not as effective in the other direction. If it holds pressure, just charge the system and move on, watching for any future leaks or performance issues.

 

Did you inspect the seals on the gauge hoses?

 

I've had schrader valves that won't hold vacuum, they need to be capped. As noted above shop air will contain moisture, make sure you evacuate the system again before charging (the longer the better) to help remove any moisture you may have introduced.

 

The shaft seal on some compressors can do that.

 

I don’t think I’d charge it up until the problem is corrected, if you can’t hold a vacuum, that means you are drawing atmosphere into the system, which has moisture in it.
The point of keeping it under a vacuum
Is to draw the moisture out.

 

Yes

 

Plan B.
I’ll let it sit with captive pressure overnight. If it holds, I’ll vacuum it & charge it and see what happens. I already put in some UV dye. What could possibly go wrong?

 

try turning the compressor over a half turn,ive had them do that,they leak somehow through the compressor.Weird but it has happened to me,turned the compressor over a bit and it was ok. Also the seals may not be "puffed" up to give a good seal,have had lots that sealed undr vaccuum and leaked when pressurized?,The new cars with all the aluminum pipes are bad for that,if the pipe is bent a bit out of shape they will leak too. harvey

 

I'd go ahead and charge her up. What's the worst that can happen?

 

I’m not an AC guy, but what kind of issues does moisture in the system cause?

 

It might need a new dryer by now.

 

Yeah...maybe. But I hate to buy a new dryer and then figure I still have charging issues. If I charge without swapping whats my worst case scenario?

 

I'd put in a can of oil first and let it pull through the system. Sometimes if the O rings are dry the oil will seal them.

 

Acid formation, corrosion, and eventually a component, like the compressor, will fail.

Pressure testing should only ever be done with a dry inert gas.

 

I store my o-rings in Ziploc bags, sorted by size, immersed in oil.

That way they are ready to go, in an instant.

 

agree with this. I've had a couple leak back through the compressor seals, during the summer they performed well but after sitting they would leak. I finally said what the hell and put in a can of leak sealant 134a. Really helped--i know a lot here won't agree with this but it worked for me.

 

Good idea! I'm going to steal that one.

I always oil mine before I install them.

 

Shop air should never be used for pressure testing , creates moisture , corrosion, acids etc in the system .


How are you drawing the vacuum and what gauge are you using ?

I would guess a faulty schrader valve or lip seal folding over under vacuum .


Is the system brand spanky new ?
Could maybe be the system is dry and just needs oil circulated through to lube up the seals etc .

never seen this before . A system should hold 30” of mercury vacuum for a minimum of 30 minutes .

tje longer you vacuum for the better as it pulls contamination out of the system , another important one that most folks miss is have good clean vacuum pump oil in your vacuum pump. The oil does get contaminated from use .

 

Moisture can condense and freeze in the metering device causing a restriction, and it also mixes with the oil in the system causing an acidic effect. Neither of these are good scenarios.


Edit: Several others beat me to the answers.

 

Great thread... Any A/C repairman would do good to comply with the list of do's and don't's in this one.
Er...There were a few steps that I wouldn't have caught, i.e. the 'rolling of the compressor shaft'!
Good thinking, @37Caddie!

 

Agree on capping the Schrader valves. They hold pressure well, but it only takes a moderate pressure acting on the bottom of the pintle to overcome the spring.

 

UPDATE- I took advice from here and tried rotating the compressor a half turn. It was stuck and wouldn't move CW. After rotating it CCW a little it now rotates freely in both directions....hmmm.
I also played wiggle jiggle with my high and low pressure couplings with the pump running and noticed a needle fluctuation as the low side was wiggled. After messing with it a bit, the system now holds vaccuum.
I'm now a little concerned about how the compressor was acting. Its a "new" never used compressor thats been under the hood free spinning for the past yeat but has never had the clutch engaged. It only took an easy jiggle to get it spinning but it makes me leary on an all new system.
So the new plan is this. I’ll buy a new life time warranty compressor for my OT Chevy pickup which also needs AC fixed. Put the new compressor in the 55 and then fix the OT truck using this compressor.

 

Ac compressors come shipped with oil in them. But it’s not AC oil.

that oil needs to be drained and the correct amount and type of oil put in the system .

usually 3 oz for a compressor
And an oz for each other part ( evaporator , confessor , drier , )


Might of just been stuck on the oil from sitting .


But I like your idea of robbing Peter to pay Paul

 

I gotta disagree with you there. The last few compressors I’ve bought, all Sanden style, came with the proper oil in them. They all had a tag on them telling you not to drain the oil out of them. Maybe it was just whoever made the ones you bought were different.

 

Same.

 

Worked as a Product Development Manager for Mobile Climate Control many many years back. You guys have pretty much covered everything. We occasionally would get a bad shaft seal on a compressor (we used Sanden exclusively for our smaller systems). At the time they came shipped with the correct amount of PAG oil for a “typical” automotive refrigerant system charge. Can’t say what they do now. I’ve seen systems leak under a vacuum. It was usually a buggered/crushed o-ring that was not installed correctly. Would “puff” out under pressure and work, but under a vacuum it would resume to the “buggered/deformed” state. Same about the Shrader valves. Should always use caps on them as they will leak a little typically. Same about using shop air. Depending on how moisture free it was, this alone could cause big problems down the road. PAG oil + water = hydrochloric acid = very corrosive. If I were the OP, I would pull a good vacuum for a couple of hours to get all that moisture out. One thing not mentioned is cleanliness when dealing with AC. This aspect cannot be overstated. The slightest contamination on an o-ring can lead to a leak. O-rings should ALWAYS be lubricated with the correct compressor oil when installed. Hoses should always be capped when new and not installed. The same goes for all other AC components. This is also true for the equipment used fir charging/evacuating the system.

 

I may be wrong on the oil , but it was a rule of thumb in every shop I worked in to drain the new compressor and fill with the correct oil and amount .

 

New ACDelco and Four Season compressors come with a sticker and paperwork stating they contain 3 oz of oil.

So quiz me this- after vacuuming the system for an hour, I disconnected the gauge set, let the hoses dangle for a bit and oil puddles up under the yellow hose. I’m thinking somehow it’s vacuum pump oil? Red and blue lines don’t dribble.