I'm having a problem trying to recharge my 66 factory AC and wonder if anyone has ever had a similar problem. It still uses R-12 so I don't want to waste any of it. The problem I'm having is it won't hold a charge. I can pull a vacuum on it, and it will pull down a vacuum to 29-30 and hold the vacuum just fine. I have let it sit with the vacuum on it for several days and it holds fine. I even checked the high and low side ports on the compressor muffler by pulling a vacuum with each on separate since they are the only thing in the system that is bypassed when pulling the vacuum. They each held the 29-30 vacuum fine for over 24 hours. I put in a recharge and also added in UV dye. I took the car out for a 20-mile drive and the AC worked just fine. Nice and cold. I checked all the AC system under the hood with a UV light and could not find any place that showed a possible leak. The next morning, I tried the AC and I got no cold air. I looked at the dryer site glass and it showed no freon in the system. It lost the charge in under 24 hours. I once again checked all the visible areas with the UV light and saw no indication of the dye anywhere. The front of the compressor didn't indicate any leak either. I realize that the only area that I can't see is the evaporator area under the dash and it could possibly be leaking. My question is why will it hold a full vacuum and not leak at all and when it is charged, lose the freon. I would think if the evaporator was leaking it would not hold a vacuum. Has anyone ever had a similar problem?

 

With vacuum you are checking the system at about 14# that operates above 200#. You need to get dry nitrogen and pressurize it. The most frequently overlooked leak is the Schrader valves at the service ports. UV should show that though.

 

The purpose of pulling a long deep vacuum is not to leak test but to dry out the system. Compressor seals are not designed to be sucked in against its intended load and sometimes leak. You want to do what pop says; pressurize it with nitrogen. Water test the Schrader valves. I've had those get me.

Dan

 

Good idea, where does one get dry nitrogen from?

 

In a pinch you could use compressed air. It should be as dry as possible. From my diving days, ordinary air is approximately 80% nitrogen. You can use a mixture of dishwashing soap and water to check for leaks as they will bubble up. Also consider buying a R-12/R134A sniffer. I bought one on Amazon for about $20 and it works very well. I went through what you did and vacuumed down my system to -30. it held for hours. I filled the system with 134-A and over a few weeks, the system was empty. It turned out the Schrader valve on the low side was bad. The system will vacuum down and hold vacuum because the Schrader valve is not utilized until the gauges are removed. Hope this makes sense. Jerry

 

I would not using air if you can help it. . Air has moisture and pressurized that would not be ideal. Nitrogen is cheap and you can get it from any welding place. One of the problems with pressurizing with Nitrogen is that it is very difficult to detect where leaks are coming from. It is probably not leaking from a Schrader valve if you cannot see the dye trace. I suggest that you pump in a few ounces of r12 and invest in a good electronic sniffer. Get a decent one, they are not that expensive.

 

Front seal. Had it happen to my A6.