i have been told I need to put a vacuum pump on the system for an hour or so before I put the r12 in. do I hook the pump to the low pressure side where the freon goes in? any advice?

The normal procedure is to use a manifold gauge set and hook the vacuum pump to the center hose...both the low side and high side valves are opened when pulling a vacuum.

If you're going to recharge it with a single hose can tapper, hook the vacuum pump to the low side fitting.

If the system has been completely flat for an extended period it would be a good idea to replace the receiver/dryer.

Evacuate the system down to 28 -30 inches of vacuum, close the valves and let is set for thirty minutes. If it holds the vacuum go ahead and recharge the system. Make sure you add refrigerant oil to the system.

thanks got a dryer and a can of oil on the way.

I have been an A/C tech for 25 years. If your system has been open longer than 5 or 10 minutes, always replace your drier. It will absorb moisture from the air if left open. Do not open the new one until you are ready to install it.