Load valve in rear brake line

[Keith Seymore]

Originally Posted by kipps

SweetK30 mentioned that this is commonly bypassed. How does this thing work?

My theory is that it's a simple valve that restricts braking force when unloaded. When loaded, it opens the valve, allowing full braking force. If true, a simple removing of the valve will bias the brakes toward the rear, since removal is the same as fully loaded. If it's removed, it seems the proportioning valve should be swapped out for a truck that did not have one of these load valves. Correct or no?

Is this valve even a problem? Why remove it?

Here's another thread discussing the issue, and evidently GM issued a tech bulletin a few years later instructing on how to bypass the valve. Keith Seymore says the valve was intended for passing a specific government test, and wasn't really applicable to the general populace. I have a little hard time believing that.

How does this valve fail? External leak only, or does it ever mysteriously block the rear brake line either on or off?

I appreciate the vote of confidence.



I did total vehicle development (including brakes) for the C/K/R/V programs at the GM Milford Proving Ground from 1986 - 1989. I did brake and driveline development specifically at the GM Desert Proving Ground from 1990 - 1994.

MVSS brake testing is so specific that some combinations would be borderline pass/fail in portions of the test procedure. We would request certain brake technicians by name to drive those tests during development and validation, just to be sure (if you don't pass the test you are legally blocked from selling trucks). Since we would "self certify" for brakes, noise passby, etc, you could also classify those as "development" tests and keep repeating them until you finally got one to pass, and then reclassify the successful run as your "validation" test. Those portions of the tests don't have any real correlation to actual customer usage, because you personally are probably not going to run a statistically designed experiment to determine if your stopping distance at GVW is "x" number of feet vs "x+1" number of feet at 150 lbs pedal force, and then trade that off for similar data at lightly laden vehicle conditions.

It's a Rube Goldberg device. Usually the failure mode is the linkage corrodes or gets gunked up and binds. If it fails it is probably going to fail in the reduced pressure configuration, and I'd rather have full brakes at LLV than not enough brakes at GVW.

I'd remove it, on top of everything else, for the simple reason I don't like any of that extra claptrap under my trucks.


K