Proportioning/combination valve re center

[Flatcap]

After fitting new rear brake lines to my 92 K1500, the inevitable happened with the proportioning valve closing off the rear brakes and now being unable to bleed them. There are a few videos on the tube but I thought I would ask you guys as to your experience in re centering the valve so that I get get the brakes bled! I have bought the tool which holds the valve in position for bleeding but stupidly not beforehand!! There is also a largish bolt head on the front of the valve & wondered if removing this could help?

You must be registered for see images attach

 

[Schurkey]

First Guess: Something else is wrong.

The "proportioning valve" does not "close off" the rear brake system. The tool you have is to center the safety switch. That safety switch is probably self-centering. Most GM safety switches are spring-loaded to the neutral position.

I've worked on a couple of Fords that had to be manually centered. A real pain in the asp.

And either way...the safety switch doesn't block the fluid passage.
If the brakes aren't bleeding...something else is wrong. HOW are you trying to bleed the brakes? Pressure bleeder? Gravity bleeding? Two-man/pump the pedal? Vacuum? Reverse injection? Something else?

Did the ABS work before?

 

[Flatcap]

Thanks for the reply. Although it's all there, I have no idea if the ABS works or not but I do know that there was no shortage of fluid to both of the rear wheel cylinders before the new parts & lines were fitted. I done a basic 2 man attempt at bleeding and gravity with no luck so far.

 

[superchevysb]

First Guess: Something else is wrong.

The "proportioning valve" does not "close off" the rear brake system. The tool you have is to center the safety switch. That safety switch is probably self-centering. Most GM safety switches are spring-loaded to the neutral position.

I've worked on a couple of Fords that had to be manually centered. A real pain in the asp.

And either way...the safety switch doesn't block the fluid passage.
If the brakes aren't bleeding...something else is wrong. HOW are you trying to bleed the brakes? Pressure bleeder? Gravity bleeding? Two-man/pump the pedal? Vacuum? Reverse injection? Something else?

Did the ABS work before?

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? It has to stop flow so the master cylinder still sees resistance for the opposite side of system to function properly. 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.

 

[Hipster]

compressed air/blow gun can push the valve back to center. If it's stuck, better to take it off and disassemble/clean out the gunk/reset on a bench. Stopping fluid flow to the side with no pressure is exactly what it supposed to do. If gravity/vacuum bled first usually this is a non-issue

 

[Schurkey]

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.

You must be registered for see images attach

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.)

You must be registered for see images attach

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.