The cycling switch will open when your pressure is below ~25PSI. Just turn it on and rotate the can upside down and right side up until you get around 2 cans in it. Getting the 3rd can in may require you raising the idle some. You really should have a set of manifold gauges when charging the system since you don't know how high your pressures are. If there's a clog or restriction in the high side you may be building too much pressure. My 88 didn't have a high pressure shut off switch (88 & 89 didn't come with one), when I shut off the engine on a hot day, restarted it 10 minutes later the pressure went so high, it blew the pressure switch out of the back of the compressor - sounded like a shot gun going off! After buying a new compressor I wired a high pressure switch (on at 250, off at 330PSI) in series with the cycling switch. No problems anymore.
Anyway, you're running a risk of blowing out your system if you don't know your pressures on both sides.