Help....is is leaking and I cant find it!

Hi,
I have a 83 300SD which has suddenly developed a leak. I have pulled the system down to 20" of merc and it will equalize back to 0 in about 2 minutes. I've checked all fittings and hoses with a Robinair freon leak detector, and still can't find the leak. Any suggestions?

Al Wong
83 300SD