What's the point of the safety shuttle valve if it does not stop fluid flow to block off flow to the area that has blown a brake line?
Any time I have a fluid leak, the reservoir for that circuit goes empty. A blocked passage for the leaking side would seal fluid in the reservoir.
It has to stop flow so the master cylinder still sees resistance for the opposite side of system to function properly.
The master cylinder does not need to see pressure in the leaking side to force the non-leaking side to work. That's why the primary and secondary pistons have the long projections on the front. See Delco service manual scan (sections 2-10 and 2-12) for details.
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Also when a front line blows and the valve shifts to block the front fluid output it Also opens a direct path for the rear fluid to bypass the proportioning valve so the rear can now get full pressure to assist in stopping in an emergency.
Seems unlikely since the only path fluid has to the rear brake is directly through the proportioning valve section of the combo valve.
I don't have an illustration scanned for GMT400 vehicles specifically, but here's a scan of how the older combo valves were set-up. The safety switch doesn't "block" anything, and there's no separate path for rear brake fluid flow around the proportioning valve.
(Figure 2-12. Click on image to embiggen.)
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The '97 C/K service manual has surprisingly little information on the combo valve. Section 5 (brakes) shows how to replace it, and Section 8C-8 tells you to look at Section 5.