Sounds like you have a major leak. Even without the compressor running you should have pressure as long as there is any liquid freon at all in the system. The pressure is temperature dependent but would be in the 80 to 100 psi range when the compressor is not running (the system stabilizes and the low and high side will have the same pressure.

You should be able to tell where it is leaking by adding freon without the engine running and listen for the leak.