Rebuilt xflow 1600 but NO oil pressure

I rebuilt my xflow completely last year and all ran perfectly after. Over this January I took the engine out (xflow 1660cc 711 block) to up rate the cam. I stripped it down to fit new cam bearings and cam, then put all back together again.
Problem is after putting oil in oil filter and engine block and on turn over no pressure registered on the oil pressure gauge.
I took off the oil gauge pipe at the tee piece on the block and turned the engine ( with no spark plugs in) and no oil came out
The oil pick up was cleaned and refitted so no problem there.
I loosened the filter and again turned the engine - no oil came out

What could be wrong?
Is the oil pump faulty ( worked last year prior to stripping the engine)
Do I need to prime the pump some how?
Is there air trapped - if so how do I get it out?

Ideas please as I am stuck

Also make sure that the cranking speed is sufficent as this can be part of the issue. I would recommend removing the plugs to aid in this process. I had a similar problem not that long ago.
Packing the oil pump is something that u should always do with a rebuilt engine.

Hope this helps.

Cheers
Craig