Ditto Club 80-90, fantastic forum.
There’s a wee o-ring in there if I remember from when I stripped and rebuilt the DG in Vinnie.
There should also be a very heath-robinson looking jubilee clip on a wee bracket that secures the dipstick tube to the water pipe that crosses behind it. If this is missing the whole tube can move in & out.
Crankcase pressure is possible – the pipe from the breather tower to the top of the carb can collapse, but usually pressure is vented via the o-ring in the bottom of the breather tower being rock-hard, not sealing,and sending oil mist all over the top of the crankcase!
Rob (& Vinnie the T25)