Sometimes, by removing the blower resistor from the housing, you can look inside the case at the evap coil and see if there is any oil staining from a leak. A/C UV dye and a black light helps with this.

A shade tree method of evacuating the A/C system, at least partially, is to make up a fitting with hoses that would allow you to connect the A/C to an intake manifold vacuum port. Use an old brake booster check valve to prevent the vacuum from dropping when you open the throttle. Under hard decel, you can get about 25" vacuum doing this. Note: If you do this, it is very important that any R-12 is gone from the system, as it makes highly poisonous gas when it's burnt!

Some old timers would salvage a compressor from an old refrigerator to make a vacuum pump out of it. Here's one link:

http://www.paragoncode.com/shop/vacuum_pump/