Metering valve/bleeding brakes

[AirSpeed]

I am trying to bleed the rear brakes on my 72 GMC, the service manual states that I need to hold open the metering valve in order to do it. The manual states that I can use a piece of tape, special tool or friend to hold it open, unfortunately there is no further explanation or illustrations on how to do this even though the manual claims such info is provided in the book!
How do I go about doing this? I am assuming the metering pin is contained in the part connected to the master cylinder via the brake lines.
My wife (and helper) is leaving soon so any fast info would be greatly, GREATLY appreciated! Thanks!! Aaron