Suction at oil cap filler

[Mach 1]

This is my first 911, bought in Nov. 2002, drove 100mi's before tearing into her for the long cold N.E. winter. repaired, replaced and rebuilt - brake system, transmission overhauled, shocks, all external engine gaskets, valve adjustment, complete clutch assy.
All work was done in my garage, by me except the tranny and now finally back together again - runs just awesome, stops great with minor adjustments to the shift coupler.
Here's my problem : added 10 qts of oil for intial start up and per manual to check my oil level is to run to operating temp, check on level surface while engine is idling, when I open the oil cap the engine stalls - all kinds of vacuum at the cap, tried starting with cap off, just stalls within a few seconds. There seems to be so much suction at the oil filler tube I could use it to vacuum my interior!
After many hours of cleaning, replacement of parts, $$$$$$ - I can't seem to check my oil level?

Any suggestions would be greatly appreciated, Mark

1986 Carrera Blk/Blk

[GB83SC]

Mark, there is a small funnel looking piece that should be in the hose that travel's from the oil filler to the air box to reduce the back pressure when the cap is opened. You can take a pair of vice grips and squeeze the hose to prevent the back pressure.
This happen to me when I droped my engine a few years back.

[GB83SC]

beware when changing breather pipes!

This should help.