Charger 9 pressure in transmission?

[Stepney 2,325]

Hello, all. I've been using a '68 Charger 9 for some odd jobs here lately, I only just got it about a month ago. It has a hydro lift and the usual 90-2046 hydrogear. It doesn't seem to leak much, besides one small dribble occasionally from a fitting at the lift cylinder. I always check the fluid before driving it and it never really looses much. But every time I remove the threaded plug, a buildup of air comes out. Sometines enough to 'pop' the cap off if you don't hold it good. My other two old Sundstrands have never done this. Any ideas why it could be? It does have a slight weep at the axles, I'm worried the pressure may blow one of the seals out. 

[Sarge 3,463]

The pressure, if it's showing up after running it for a while is from heat expansion. The only vent on those is built into the dip stick, make sure it is clean and open. 

 

Sarge

[Pullstart 63,097]

Maybe try that dipstick on another auto to see if that’s the culprit?  Maybe it’s obvious already...

[Stepney 2,325]

Honestly, I never even noticed a vent on all 3 of mine, I'll take a look. Feel kind of daft now haha. 

[pfrederi 17,885]

Make sure the hole in the bottom of the plug is clear and the nodular brass filter is not painted over on the top.