Without bleeding the master cylinder first, All the pedal pumping will be wasted energy.
Once its bled you easily continue. But even after that, pumping the pedal with all the nipples open, will just cause frustration. Open one nipple (closest first) press the pedal and hold it in the down position, then close the nipple and release the pedal. You are relying on vacuum effect while there is more air than fluid in the lines.

Why leave the pedal partly depressed?
Benny's right, this pushes the piston into a position that cuts off all hope that fluid will run down under gravity.

You would need the pedal in the fully up position to allow the resevoir to access the cylinder & for fluid to run down the lines.

keep pumping............It eventually fills. or use one of those funky pnuematic brake bleeding tools [basically creates vacuum and draws the fluid through]. easy one man job.

Having the pedal partially pressed moves the plunger past the hole that the fluid enter in the MC and closes the cct.

New front brakes (SR20) & new rear brakes (R31).
Everything's been apart & cleaned, but now I have to prime the system somehow so I can bleed the brakes.

I left the rear brake bleed nipples unscrewed last night & the brake pedal partially depressed in the hope that gravity would do it's work, but if it doesn't I'm out of ideas.

Perhaps the mechanics out there have a device whereby they "suck" the fluid down through the line via an attachment on the brake bleed nipples?

Anyone had experience with this?

Thanks in adviance,

Coops