Brake problem

[Stout72]

The front discs on my '72 squeak to the point of embarrassment. A quick check thru the rims revealed there's plenty of pad left, and the rotors look ok. I figured I'd begin with the easy stuff first and bleed the brakes. The forward reservoir in the master cylinder was way low (tracing the line, it leads to the rear brakes... got a leak there somewhere). Reading up on the subject, I learned that the combination valve must be held open during the bleeding process by holding in the metering pin. I'm not sure where this pin is. If it is on the forward end of the combo valve body, mine appears to be stuck in the "in" position. The end of the pin is flush with the boss thru which it protrudes. I'm new to this type of brake system, incorporating a combo valve. What's my next step?

------------------
72 Chevy K20 LWB, 350/350, lifted, dual shock suspension F&R.
AKA The Beast. Daily driver, even though it's a work in progress.