Dave- what are you using to draw the vacuum? I have one of those $15 harbor freight venturi pumps, but it doesn't seem to work very well.
I have the same thing. It pulls 25 inches using my 6 hp compressor (6.4 cu ft @ 90). Anything over 10 or 15 inches should do ok. Just might need to pull longer if you can't get a lot of vacuum.
Also, when I first begin pulling vacuum, I shut down the high side and low alternately a couple times just to move any moisture there might be in the furthest recesses. Then I let it suck for 20 to 30 minutes.
BTW, still rainy here.