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The quick take up function serves both the front and rear brakes because the volume of fluid delivered across low pressure MC chamber is enough to drive the secondary piston as well as the take up for the primary circuit.
If you go to this link and download the "Base Brake Systems" it'll walk you through all the functions.
https://www.gmt400.com/threads/88-95-service-manuals.43575/
I guess - there is a ton of conflicting information on the forums. Folks are putting random master cylinders on their trucks, residual valves, all kinds of nonsense.
From the service manual for a 94 it says this:
"The master cylinder (figure 1) is designed for a system using low-drag calipers. In addition to the standard master cylinder functions, a quick take-up feature is included on models with a vacuum booster. This provides a large volume of low pressure fluid to the wheels with the initial brake application. The large volume of fluid helps overcome the clearance created by the caliper pistons and rear brake shoes retracting."
I guess that's why the pedal sucks on these, huh? The big "squish" before much happens is because of the QTU function.
My issue is I did an ABS delete a year ago (the master, both front brake hoses and one rear brake piston were replaced at the same time), and if I really mash the pedal, I get a brake warning light. I've had it bled multiple times... As far as I can tell, the brakes themselves work fine, but the pedal is still mushy and it triggers it. What's throwing that combo valve out to the point it triggers that light?