Before you start worrying about blow by, look at this:

YouTube - 1985 Ford 6.9 IDI Blow By Take Two

That video shows normal blowby at the filler cap and thats nothing to worry about. If you can feel actual pressure under there while the breather system is hooked up, then you might have a problem. One of the old school ways to check for how much is too much is to leave the filler cap in place but not turned to the lock. If the blowby pushes it up, then its too much.