yeah, I don't know about the other prototypes but the D1G ball was kept at a lower pressure then ambient (I suppose any "leaks" were into the ball, not out)
Every frickin time you went in or out of hull you had to go through an air lock that would pop your ears a bit... that is where I learned how to equalize at will...
kinda funny, that was about the only thing I learned on that surface plant that helped me on subs