Does it have any pressure on the high side? I've found in a couple vehicles in the past that they would not pull any freon on the low side until the high side got "jump-started". If the high side has any pressure at all DO NOT OPEN THE HIGH SIDE VALVE. But, if it's still in a vacuum or no pressure, dump a little freon into the high side. It might be just enough to get the compressor engaging and pulling freon in on the low side. Again, NEVER open up the high side valve if there's any pressure over there. The pressure will backfeed into the freon can and turn it into a grenade, complete with schrapnel. If in doubt, don't do it.