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Replacing hoses and o-rings may be a good start in reviving your car's air conditioner. It's a pressurized, closed system that lives on an expensive gas held in place with flexible rubber parts that seal less well with age.
But before you get started, it might be helpful to know more about the system you're about to repair. If it's the original AC system and you're already thinking about a new compressor, high-side hose(s), and all o-rings, that's already a long way toward a complete overhaul.
The compressor is the one moving part in the system and the most expensive item you're probably looking to replace. The rest is plumbing. If your last compressor had a complete meltdown or if some moisture got mixed in with the refrigerant, then there could still be enough junk in the rest of the plumbing to kill your new compressor. When you open an AC system, you want it as clean and dry as possible on the inside before you seal it up again and recharge the refrigerant. If your system has had recent work or still holds a partial charge, then you might do well with a few new parts and a thorough vacuuming to pull out all the old refrigerant and any moisture.
If your system does still have a partial charge, let the pressure down gradually through a gauge set on one of the service ports before loosening any hose connection. (A professional shop would collect the old refrigerant to keep it out of the atmosphere.) A sudden drop in pressure, like that you would get if you took a hose loose, will send the oil in the system all over the place. That's a cleanup problem for sure, but also the system requires a certain amount of a certain oil. When changing only the compressor in an intact system, for example, you'd want to measure how much oil comes out of the old compressor and put that same amount back in with the new compressor. That oil's hard to measure when it's all over the wall.
An intact system is always under pressure. When the compressor is idle and the system is at rest, the refrigerant pressure equalizes so that "high side" pressure is the same as "low side" pressure, but still way above atmospheric pressure. When the compressor runs, it draws refrigerant from the low side and pumps it to the high side. The purpose of the compressor is to create high pressure in the refrigerant (condensing gas into liquid) which then sprays back to low pressure (expanding liquid into gas) through the expansion valve or orifice tube at the evaporator. Inside the cabin, the expanding gas inside the evaporator is where the chill occurs; out in front of the radiator, the transfer of heat to the atmosphere by the condenser helps the refrigerant gas shrink back to a liquid.
On my system, there are three high-pressure hoses and one low-pressure hose. My high side hoses are these: compressor to condenser, condenser to drier, drier to expansion valve. The low side hose leads from the evaporator back to the compressor. The low side hose is larger in diameter than the high side hoses and connects to the compressor at the fitting marked "S" for suction; the high side connects at "D" for discharge.
You may know all this already and I don't mean to presume otherwise. But maybe this will help you get it right the first time. It's no fun having to do the same repair twice.
Good luck with it!
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