Franc, sure it's possible to have Zero pressure. All leaks will eventually result in that if given long enough.
However, since you have retrofit connectors, they may just not be interfacing properly with your machine, and depressing the valve core pin properly. A quick check is to just momentarily press one of the valve pins on the car and see if you have any pressure on the system. If you have any freon at all, you should have about 90 psi at an ambient temp of 80 degrees F on a non-running system. That should be eassy to tetect with a quick punch on the valve core.

I take it you found that sneaky little high side port.

DG