The AC isnt working since last fall. At that time they changed the evaporator valve but we couldn't tell if it was working because the weather had turned colder. Now it's warming up again and the AC doesnt work.
They tried putting gas in it and it still doesnt work, so they said they should drain it to a vacuum, refill it so there are no possible air bubbles, and then see. Only their machine couldnt draw a vacuum, which they said means there may be a big leak somewhere?
So they said they need to bring over a better machine and check it again.
The story is plausible. If the system has been exposed to the ambient atmosphere for a while, then you will also need to replace the receiver/drier because the desiccant bag in the receiver/drier will be saturated with moisture and will not be able to absorb any moisture in the system.
That only applies to a converted R-12 system though. The condenser coil is undersized with a R-12 system converted to 134a, and that leads to higher than normal head pressures, which in turn lead many to slightly undercharge the system to keep the high side pressure down. But a system that was designed for 134a does not have this problem. The condenser coil is properly sized, and you don't run into these issues. The 134a system in my 05 Corolla will turn you into a popsicle. But I agree. An R-12 system is best with it's original R-12, and that's always what I do if I run into that. IE: my previous Honda Accord was R-12, and I kept it that way. I have a thirty pound jug of R-12 about 15 feet away from me sitting in a corner. :) I use it for the R-12 cars I might run into, and also older R-12 refrigerators and freezers.
Sounds like you need a new A/C shop to tell the truth.. But there is really not enough info to make a very good judgment. If the compressor would run when turned on, it's obviously not low enough to kick in the low pressure switch. If the compressor would not turn on, then it could be very low. If they changed the evap metering valve, whatever type it is, they should have also changed the drier. If they didn't, they are not really with the program. If the system was never totally empty, there is no need to draw a vacuum and recharge. As long as there is any pressure in the system, even if only five pounds of pressure, moisture can not enter the system. Only with a *totally* empty system, with the pressure equal to the outside, "0", can moisture enter the system. But like I say, I don't have enough info to tell if it was totally empty or not. If the system has a leak bad enough to keep it from holding a vacuum, then yes, in most cases the system was totally empty. And it will need to be vacuumed and recharged after they find out what the real problem is. But they need to find the real problem, and not just throw bandaids at it. I'd almost bet they didn't change the drier the last time, and it's clogged up. But that would not cause the system to leak. A leak would be a separate problem. Anyway, would need more info to really make any accurate guesses. That they put gas in it, tells me the compressor must have been running. You can't gas charge a non running system. So if it was running, and charged as they claim, that points to a restriction. Either in the valve they replaced, or with the receiver drier. Or both.. Need more info.. If it has a leak so big as not to be able to draw a vacuum, that's a BIG leak, and should be easy to find. But I hope they are using a real vacuum pump, and not some junky converted refrigerator compressor converted to a vacuum pump. Those do not draw down far enough to boil the moisture out! And moisture can freeze up and cause restrictions.
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