check the electrical before you do anything!
trust me. i wasted 600 dollars and then found out it was the relay!
my compressor was doing the same thing. the relay wasn't all the way bad and when we squeezed it the a/c worked and the compressor clutch engaged. it's a 5 dollar relay and not worth overlooking.

your pressure is low on the high side because the compressor is not kicking on.
if there is no pressure whatsoever, you may have a leak. if you are too low on R12 the compressor will not engage.

 

You found zero pressure, this means there's a major leak. You're just going to have wasted the R-12 putting it in without fixing the leak first. Test for leaks by putting the system under pressure with nitrogen, R-134a, or air (Maximum test pressure 150 psig, and do not run the compressor). You can find them by listening for the gas to hiss out, using soapy water, or if you used R-134a, an electronic leak detector. The condenser is a likely suspect, it could be corroded out or damaged by debris from the road.