mmmmm a tricky one
whats the cranksharft like is it nice and smooth and straight and is the seal in the wright way round.
when you get it going again try leaving the oil filler cap off for a while. is there any end play in the cranksharft, maybe its moving back and forward when you use the clutch and dragging the seal backwards. I don't think its air pressure as the dip stick would lift out first.

Pre-check time.....Do you have the correct:-

Rear seal (obvious I hope - but cheap fix - was the crank reground and have you allowed for a new seal size),

Crank (big end) Bearing set (is the crank moving excessively in a north/south direction when the clutch is applied - cheap fix),

Is the crank bent or twisted (high cost - a possible twist or bent could be driving the seal of the crank...sort of like a screw thread),

Are all the bolts to do with the seal, crank and flywheel to correct torque settings - if one is out it could be putting a miss balance in the system. Did you Lock-tite them in place - they do vibrate there way out doing horrible things to high reving motors.

When you installed the seal did you lube up the seal and the crank face before starting the engine?

Next time you start the engine, do it with the dip stick out - this will garrantee there is no sump pressure (I really don't think pressure is your problem)

These might not help, but then again they might
Micko

on second thought it sounds like the seal is in backwards or the wrong seal to start with.

thank you all very much, you have given me heaps of things to check, when I finally get the thing together again I will let you know what the cause was.

Hi just wondering whether there is any one on hear yet who has encountered this problem - I'm nearly ready to put it back together!

Sorry I can't offer any advise on the seal popping out, but before you reassemble it, have the head hardness tested. Once they go soft, they wont seal worth a dam.