4 speed leaking rear seal??
The boot is probably 1/16” or slightly less on the yoke.
Not sure what to make of this, but it doesn't sound good. What it does sound like is that your driveshaft is waaay too short.
There are two types of seals, one is like the front pump seal in an automatic. The second has a dust boot integrally mounted.
If in doubt, disconnect the rear, and push the shaft as far into the trans as it will go, then pull it back out about 1 inch, or just until the dust boot is free. If the U-joint caps are more than 1/4 inch from being seated in the rear yoke, then the shaft is too short.
On a different note, make sure the vent is breathing. If you like, you can pop that piece out of there, or pop the top off, and pressurize that box with shop air regulated to no more than 4psi. That will find/prove your leak right quick.
