No rear brakes

[Schurkey]

but when you look at the service procedure for replacing the valve, it says that the metering rod has to be held in during bleeding.

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I'm guessing that holding this pin down prevents flow into the front lines by keeping the metering valve closed, increasing pressure and allowing the differential piston to re-center and allow fluid into the rear lines.

Two mistakes here.

1. The metering valve is intended to block fluid flow to the FRONT brakes. It has nothing do do with the rear brakes. Holding the pin in is to allow--not prevent--fluid to flow to the front. The only time this is required is when PRESSURE-BLEEDING the system; and only then when using moderate pressure. Low pressure isn't enough to close the valve, therefore there's no need to dick with the pin. High pressure is enough to pop the valve off it's seat, so there's no need to dick with the pin. But moderate pressure will close the metering valve which blocks fluid flow to the front brakes, so the pin has to be moved manually to get the fronts to bleed properly.

2. The differential piston doesn't block fluid flow. It activates the safety switch, but doesn't seal-off any passages. Most differential pistons are self-centering on GM. I've had a couple of Fords where they had to be manually reset.

 

[Jack97]

is any fluid suppose to come out of the pin if it is pressed while pressing or depressing pedal?

 

[Schurkey]

No.