1994 Chevy Blazer AC Clutch wont engage

I replaced the R-12 with R-134 worked great for one summer then quit getting cold checked the freon and it had leaked out I fixed the leak put more freon in it an now the compressor cycles for a split second the shuts off I am running at the right pressure 30 to 40 psi my question is do I have enough freon or do I need to put oil in the compressor or is there some switch or relay that has gone faulty thank you for you help if you can any advise would be helpful.