I am not sure if this should be under tech assistance or here, in the A/C Forum. But since it IS a weird problem, at least to me, I figured the answers might be useful to others doing general A/C repairs now in the Summer.

Time to ask the experts. I am running out of hair to pull. This is on my own truck. Last summer the A/C suddenly died while driving out of state on a really hot, muggy day. Of course. When I got back home, I checked out the system and confirmed that it was very low on refrigerant, but NOT completely empty. Decent leak somewhere, but NOT "open" system. It had worked fine for years up until then. It didn't even need periodic topping up of R-134a since I put the engine in it about 10 years ago.

The only leak that I have ever seen on it was where the suction hose connects at the receiver drier and the quick connect on the liquid line just before the evaporator case. All other connections are dry. The bottom of the evaporator case was really oily, but I wasn't sure if it was from the engine or a/c oil, so I removed the evaporator to inspect it since the lines were both already off anyway. It looked like a lot of the oil was actually from the leaking suction hose connection after getting the case and evaporator core out, but the evaporator itself was very clean, with NO signs of ANY leaking. After replacing the o-rings at the previously mentioned locations and reinstalling the evaporator (ya gotta REALLY appreciate this old design for the ease of access to EVERYTHING!), I hooked up the vacuum pump and tried to evac the system. No joy. Maybe 20 inches of vacuum on low side, same on high, but when closing the high side service valve, I got a little more vacuum.

Since the truck only gets used for occasional towing and hauling mowers or engines, I decided to put off any repairs until later. Fast forward to THIS Summer and covid/normal Summer slow work. I decided to go ahead and try to find out what is the actual leak.

I replaced ALL of the rest of the o-rings in the entire system since these had such a bad reputation for leaking anyway, even though those joints showed no signs of ANY leaking, and I replaced the suction/discharge hose assembly since the high side service port was bad. It had one of those stupid non-repairable rubber ball valves that leak instead of a real schrader valve. I had used a threaded plug for a semi-permanent plug for many years in the bad high side service port. Everything hooked back up and all joints lubed with Nylock(?), the gooey, clear sealer that is supposed to REALLY WORK. It certainly costs like it should work anyway!

With the vacuum pump going, I can still only get about 25 inches of vacuum! AAAHHHH! Now what!? If I close the high side valve I can get maybe another inch or two of vacuum. As soon as I close the low side valve the vacuum immediately drops to about 7 or 8, then slowly drops the rest of the way to almost zero. If I pressurize the system to about 110 psi with shop air, it will hold pressure for over 12 hours. Something is working like a one-way valve: stuff can get in but not out easily. This does NOT sound like a "hole" in something, otherwise it wouldn't hold pressure either.

Due to the fact that the low side holds a tiny bit more vacuum than the high side, I am guessing the leak is somewhere in the high side, between the reed valves in the compressor and the orifice tube. But I can't find ANY evidence anywhere! The system had dye put in it years ago, long before I got the truck, and it is still in there. NO green spots, no oily spots, except for the original leaks near the evaporator. The lines are all OK, and the condenser is totally dry. The compressor shaft appears to be totally dry.

Until now, I have always been able to find leaks by looking for the dye/oil stains, soap bubbles and sometimes by smell for bad evaporators with R-134. I have never had much luck with electronic sniffers. I have one, but I haven't used it in years because it doesn't seem to detect even obvious leaks very well.

OK. Flame suit on. What on Earth am I missing? It "should" work. I think. But I am starting to question my own sanity now. Any ideas? Compressor shaft seal? Condenser has invisible leak?

Sorry for the novel!

John Jagitsch
Owner/Technician
TJ's Auto Service
Champaign, Illinois, USA