Replaced CDR valve. Old one was shot. New one came with seal between valve and intake, but no o-ring for the junction between the CDR valve and crankcase tube. I put it in anyway and it seemed a snug fit. Is there enough pressure there to really need one?

 

There is no real pressure or Vacuum there just air movement, at a idle the pulses of the pistons moving and the intake pulses work to positively vent the crank case, when you remove the air cleaner it wont work as well as the very slight restriction of the air entering the intake make the valve work right.

 

My 1989 7.3 IDI has a rubber tube that snaps into the valley pan on the lower end and slips over a metal tube on the CDR at the upper end. The factory crimped a metal ring around the upper joint.
There is a square o-ring between the CDR and the intake manifold.
I'm not aware of any "crankcase tube".

 

By crankcase tube I meant the valley pan tube. It vents crankcase gasses, no?
Yeah I think its not enough pressure for me to worry about finding an oring for the metal tube. The CDR valve is bolted on and holds the rubber tube down just fine, putting enough pressure on it that it makes a good enough seal, I reckon.

 

By "crankcase tube" I thought you meant something like an old road draft tube.

I didn't put any kind of clamp on mine when I replaced the rubber tube. I think you mean "clamp" when you say "oring".