this is what I had in mind. Cut/paste the url into your browser.

"http://www.harborfreight.com/25-cfm-vacuum-pump-98076.html

The venture pump is the vacuum level will not remove moisture as well, but venturi pump is better than nothing.

Suggest removing and inspecting evaporator. This is likely source of leak.

Also check compressor. If there's a large dirty oil/dirt ring around compressor clutch rotation plane, the compressor shaft seal is leaking. Shaft seal can be replaced or compressor replaced.

good luck