1986 C10 not holding Vacuum

[BLong]

First time to post but very glad to have found this forum.
I have a 1986 C10 that I am trying to get the original AC system working. It worked 3 to 4 years ago but someone took the evaporator out. I have flushed all of the lines and replaced the Evaproator and Dryer. After replacing all of the o-rings i hooked everyhting up and it held a vacuum. then the compressor did not work (I did not replace). Replaced Compressor but now I cannot get it to hold a vacuum. I used the metal washers with rubber seal that came with the compressor.
My question is any advice on how to see where the leak is without haveing refirigerant in the system? I have heard of using compressed air and listening. I am replacing all of the o-Rings agian to see if that helps.
thank you
Bryan

[JohnHere]

Welcome to the Forum.

I don't advocate the use of compressed air to check for leaks. Air also contains moisture, neither of which you want in the system. Instead, I would charge it with a few ounces of pure refrigerant to raise the static pressure to 20 or 30 PSI. If the leak is a big one, you should be able to hear it. If not, and you already have--or can borrow--a refrigerant detector (sniffer), scan or "sniff" all around the compressor and its connections. Since the system held vacuum before you replaced the compressor, I would concentrate my leak checking efforts in that area first.

Is the new compressor an OEM unit or a rebuilt? Rebuilt compressors can sometimes leak and give other problems.

[BLong]

John, thank you for responding. It is a new compressor. the old compressor had used a rubber seal that seated and sealed well in a groove. this compressor is a flat surface. I will try the refrigerant and I do have a sniffer but I am not sure how good it is.
Thanks Again,
Bryan

[JohnHere]

John, thank you for responding. It is a new compressor. the old compressor had used a rubber seal that seated and sealed well in a groove. this compressor is a flat surface.

I suspect that the leak is where the two refrigerant lines, joined by a single flange, bolt to the back of the compressor. If that's the case, there's probably a mismatch between the original flange and the new compressor, which as you pointed out uses a different style of sealing. The solution might be to replace the original line/hose assembly with a new one, but I'm not certain about that.

GM used two different compressors on your truck--the A-6, a long, slender, heavy unit having a clutch about the same diameter as the compressor housing; and the R-4, a short, stubby, lighter-weight unit having a clutch that's smaller in diameter than the compressor housing. I'm not sure which one your truck has, but IIRC the connections on the backs of the compressors are similar.