Most time its pressure in transmission causing leak at shift shaft so no venting. The vent in looseness off plunger in transmission cover that valve looking spring hole engine up off backup disc air goes in and out by plunger my 2 troybilt plungers were frozen up and shifter shaft leaked after free plunger and adding transmission fluid leak stop and yes take shaft from inside to change seal. There way to make vent by taking T handle out go to grinder cut straight up grove in threads so vent by threads. You drill case cover tape female pipe thread add rear end vent to cover off car or pickup this takes more tooling.