If your going to do any a/c work I suggest investing a set of gauges. There are cheap china sets for around $40-60 that will do the job for the average diy'er.
The following link shows the static pressure of the system based on the system temp prior to running. Both the low and high sides have equaled. The can/gauge setup will show high pressure at this point.
http://www.csgnetwork.com/r134apresstempconv.html
This next link shows the comparison between temp and pressures. Pressures will vary between manufacturers.
http://www.ackits.com/aacf/ptchart.cfm
I'm assuming that your using the can/gauge setup that most places carry. This only gives you a lighter wallet. What may be happening is that the can is raising the low side pressure enough to engage the compressor. The compressor runs but reduces the pressure on the low side & the low side cut out switch cuts the compressor off. Pressure builds back up and the compressor starts again. This cycle repeats over and over. This is happening from a low refrigerant level.
Another possibility....
There is a restriction on the high side. The compressor builds up pressure until the high side cut out switch disengages the compressor, pressure drops and compressor starts again. Cycle repeats just like the above scenario.
Again, we need to know what the pressures are.
A lower cost method of determining whether the problem is on the low side is to jump the low side pressure switch. If the compressor disengages while running the high side pressure switch is disengaging it. DO NOT jump the high side switch - can get costly if you do. These compressors can push over 400 psi into the system.
There is another possibility of air contamination, unfortunately you need gauges to determine that. Since you stated the system was low this may also be adding to the problem.
The system switching from vent to defrost may be vacuum related. Others may chime in on that.
