A/C Compressor

could be alot of things. clutch in the compressor might be bad. switch could be bad and yes no/low pressure could do it. you need to find out if it will hold pressure first and also try to figure out if the compressor is good. if its all original, you might as well change all the lines and seals and upgrade it to r134 refrigerant. If you are insistant on doing all this yourself, there are some good books on a/c out there. {google or E-bay}. that's the
best I can tell ya, I am not an a/c expert but thats what I would do.