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From my limited experience with common Gast vacuum pumps, they are dry sliding-vane pumps that draw nowhere near the low pressures you want for AC work. You are looking for an oil-wetted piston pump set, preferably multi stage, with inlet pressure measured in microns rather than inches.
Look for a two-stage Robinair or similar quality pump, it will come with the correct hose fitting. Get extra oil too; over time the oil gets moisture contaminated, and the pump won't pull below the boiling pressure of the water in the oil. You need it low enough to boil water out of the oil in the AC system. So new vacuum pump oil every few cars.
I have to 2nd Dr. Bob's advice. I recently bought a Robinair 2 stage pump from Tooltopia at a very good price and I believe it would suck the chrome off a trailer hitch ball.
I tried two HF Pittsburgh vacuum pumps. While they pulled vacuum both released vacuum after the pumps where turned off after 5 minutes. I would have to insert a valve in line to prevent that.
I used Dave Davie's GAST pump earlier this year and was pleased with the results. The GAST pump I am buying pulls to ~29 of vacuum.
That looks exactly like the pump I bought, although I don't think it was from HF. I had no problems with it releasing vacuum. Just to test it and the gauge fittings, I pulled a vacuum on the manifold set and held it there for several days. My car didn't hold the vacuum but it wasn't the pump's fault...
That HF pump looks like a goody. Isolate at the gauge set before you shut off any pump. Then admit a partial charge, suckerdown again and then do your final charge. CFM is not important, the quality of the actual vacuum is. You can get by using a non-refrigeration type pump like a gast but you may want to do three flushes to get a high quality charge. Moisture is the biggest enemy, so letting it run for a while will dry out your system.
It must pull enough vacuum to a) boil the moisture out at current temperature. The moisture is in the oil so it's not as simple as just pulling it off of bare metal tubing. It also needs to b) get the air out if you want good system performance. 29" won't do for these duties.
Moisture in the system fights you in a couple ways. It causes actual ice formation inside the expansion valve, limiting the valve's action as well as the flow of refrigerant. Moisture also reacts with the refrigerants to produce an acid that loves to eat through aluminum parts. Since the condensation seems to happen at the coldest parts of the system, you get corrosion at the bottom corners of the evaporator and the condenser.
Air in the system, even a small amount, confuses the expansion valve. It also clogs the condenser limiting its effective capacity, uses compressor capacity, and offers no cooling into the evaporator. It actually carries heat into the evaporator. Doesn't take much air at all to turn a 30º system into a 45-50º system, while forcing the system to work harder doing it.
Vacuum pump is an area where it doesn't pay to go cheap. Get a good micron-level electronic vacuum gauge too. I can suck the low-pressure gauge needle hard against the stop pin, and still need another 12+ hours to get the vacuum low enough to call the system dry and empty. Moral- don't rely on the manifold gauges to measure vacuum.
06-06-2015, 10:58 AM
Outfitting a Vacuum pump for pulling A/C vacuum
but it wasn't the pump's fault...