question for transurgeon

I have a 1978 F-100 with a C-6 tranny, rebuilt less than 100 miles ago by someone who normally does damn good work on GM trannies. I noticed a wicked slip on the 2-3 shift yesterday while driving home, checked fluid, its fine, so I crawled under the truck to check the modulator valve to make sure it was tight, hose was ok, etc., and when I pulled the vacuum line, transmission fluid ran out...is this normal? If it turns out to be just a bad modulator valve, that sure would beat having to pull the tranny for a 3rd time. If it isnt, I think I might have to find another transmission mechanic....where is your shop at again???