yep, on the 134a car the high and low side hook ups are different sizes. Alot of cars have red and blue caps on the hook ups. R12 cars are a different story, they weren't dummy proof and you had to be more careful with the connectors. A/C systems work basically the same in a house as they do on a car. I pretty sure your right David, you have to charge the system on the low side with the refridgerent in liquid form. It has to be a gas to be on the high side because a liquid won't compress and you will ruin your ac compressor. Hope this helps. Alot of car stuff is hard to explain over the computer.