Hello, is there any moving parts in a 68 power brake proportional valve? I bought a used one and it does not appear that there is any piston that can be removed for service? i assumed there was something that shifted and came in contact with the brake switch if one side lost pressure? Thanks - RB

 

Here is a little bit on the subject

https://www.corvetteforum.com/forums...ing-valve.html

I managed to disassemble one a little more but don't have a picture. It can be taken apart further to get at the slider.

Unless you have a pristine example and can verify its operation, a part that inaccessible calls for a fresh start with a new part.

 

You have the 'normal' distribution block for that model year. It is there to isolate the front brakes from the rear brakes [or vice versa] in case of a significant brake fluid leak. Thus, at least one-half of the system is still operational; and when the shuttle valve in that block shifts from its centered position, the BRAKE light on the speedometer is lit (to tell you there is a problem with the braking system).Last edited by 7T1vette; 05-27-2017 at 10:28 PM.

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Rookie guess here: It seems to be what 7T1vette describes as a Distribution Block/Valve rather than a Proportion Valve.

Here are pics of my 1968 Distribution Block/Valve......






Dash warning light switch removed....



There is a beveled brass orifice on each end...use an aluminum dowel rod to tap them out from the opposite end so you do not scratch or damage the surfaces,,,,,



Clean all the small parts and coat with brake fluid before reinstalling them....MAKE SURE the piston is centered inside the body before screwing the switch into the bottom of the body



The two o-rings became gummed-up and would not allow the piston to center itself.
I honed the bore of the body out/cleaned all the parts/reassembled the assembly.
Since the piston needs brake pressure to move in either direction, it will divert brake fluid to the system that has the highest pressure-either the front brake system or the rear brake system, and will block off flow to the system that is leaky/blown seal/rupture brake hose/etc.
As the other members said..this will help you get the car to the nearest repair shop with either the front system or rear system still working. (I think!).

 

thank you all! that was EXACTLY what i was looking for. have a beer on me!

 

This block doesn't stop fluid from the leak side, it only operates to light the brake light if one side leaks.

 

These guys are the only ones selling a rebuild kit I have found. They also have pictures on how to pull it apart.

https://www.musclecarresearch.com/gm-pd-valve-kit-4