Proper "burping" of a transmission...

[tex_waco_usmc]

86 GMC 350 TBI/700R4.....

Was wondering what the proper technique is for "burping" a transmission when installing. I thought I did mine correctly but I have transmission fluid under the truck now and when I looked it is dripping off of the driveshaft behind the front U-joint. The rear seal is new so I know it isnt that (plus there is no evidence that it is coming from the seal). The only place that makes sense is the "pressure relief" tube on the top of the transmission...

And...if it is coming from the tube, should I be expecting a grenade of the transmission soon??? Currently it appears to be shifting out fine...