What is Vacuuming the A/C system, and why do I need to do it?

When you vacuum an A/C system, you're not cleaning it, you are removing AIR and MOISTURE.  A vacuum pump is connected to the yellow hose of a manifold gauge set.  The pump pulls all of the air out of the system.  Also, water 'boils' in a vacuum, so all the moisture is removed from the system.  Air and moisture are harmful to an A/C system, because they react with refrigerant, and create an acid that can damage or destroy seals and o-rings.

A vacuum is pulled AFTER all the components have been replaced and the system is sealed up, but BEFORE refrigerant is added.  Typically, the vacuum pump should be left running for about 1 hour, longer in especially humid climates, or when the A/C system has been left open to air for an extended amount of time.  A vacuum must be pulled anytime the system has been exposed to air, or had a toal loss of refrigerant.

Most vacuum pumps will only pull a vacuum of 28 psi max.  26-28 psi is fine.  I normally let the vacuum run for about 15 minutes, then I shut the valves on the gauges and watch for a minute or so.  If the needle on the low (blue) side starts creeping back towards zero, I know something is not correctly sealed.  If it holds solid, I will re-open the valves and let the pump run.

While vacuum pumps are pretty expensive, many retail parts stores rent or loan them to the do-it-yourselfer.  There are less expensive, but less efficient models that run off of shop air, and those will work fine for the ocassional odd repair.