bad a/c compressor

[Racincc85]

the other day i noticed my a/c wasnt blowing as cold as it did a month ago. i hooked a set of manifold gauges up to the system and the high side pressure was about 150 psi below what is should have been and the low side was down about 10. I thought it might be low on freon, so i hooked a 12 oz can of 134a up to the gauge set and tried charging the low side. There was hardly any suction and it took about 20 mins to get half a can in. The pressures didnt go up either, so im assuming that my compressor is bad. Anyone else have ideas? I dont want to spend 200 bucks on a new compressor to find out it wasnt the problem. Also, i retrofitted the ac system to 134a the year before. the car is a '90 buick lesabre limited. thanks for the help!