break qustion

[Josh577]

hey guys im working on a 84 GMC k20 with my son, this is my first go around with one of these trucks.
we have the bed off doing the support rails and i figured we would do the breaks why the bed is off.
in the image im going to attach is this mechanism attached to the rear end, that I’m assuming is a proportioning valve but i don't know. i cant find any things on line that looks like this thing can any one tell me if it is what i think it is?
can i rebuild it, can i delete it, does anyone know of a suitable substitute?

 

[Turbo4whl]

Yes that is a proportioning for the rear brakes. When the truck is empty (lighter and higher) it limits the brake fluid to the rear. When the truck is loaded, the truck lowers in the rear, then you get maximum braking.

This is to keep the rear brakes from locking up when the truck is empty. Some people do remove them and put in a manual adjustable valve.

 

[skysurfer]

Load Sensing Proportioning Valve. You can enter LSPV in the search bar and it comes up in quite a few threads. That one looks like it has seen better days and should probably just be bypassed.

 

[Josh577]

Yes that is a proportioning for the rear brakes. When the truck is empty (lighter and higher) it limits the brake fluid to the rear. When the truck is loaded, the truck lowers in the rear, then you get maximum braking.

This is to keep the rear brakes from locking up when the truck is empty. Some people do remove them and put in a manual adjustable valve.

thank you brother

Load Sensing Proportioning Valve. You can enter LSPV in the search bar and it comes up in quite a few threads. That one looks like it has seen better days and should probably just be bypassed.

thank you brother

 

[dvdswan]

That is the Load Sensing Valve. The Proportioning Valve is on the very front cross member and looks like this...

1984

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1974

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[Josh577]

i that load sensing valve took off tonight all in one peace, you guys think any resto people would want that thing?

 

[squaredeal91]

i that load sensing valve took off tonight all in one peace, you guys think any resto people would want that thing?

Possible. Some say trash it others say keep. In my youth I'd toss, but now I'd probably keep it for a while and maybe toss later.

 

[fast 99]

GM is cheap. They don't install parts if not needed. Carefully analyze any brake modification.

Side note, man you eastern folks have to deal with a lot of rust. Don't think many here would even consider fixing a vehicle in that condition.

 

[Keith Seymore]

I can add that that height sensing valve was a Rube Goldberg-esque attempt at a mechanical anti lock brake system, before there was an electronic version.

It was specifically added to help meet MVSS Federal standards for stopping distance. Some test scenarios (any of of a combination by model, brake series, wheelbase, rear axle, tire type and curb vs GVW) were so tenuous that we would ask for a specific driver to run the test. We would run multiple tests and classify them as "development" until we got a pass and then categorize that one as "validation" and submit it to the Federal Government.

If you are not trying to meet federal stopping distance requirements by within a few inches then you could probably leave it off completely or, as suggested above, add a manual adjusting prop valve back there.

K

 

[Grit dog]

If it’s hooked up and actually rotates, then it works. It’s just a simple bypass valve that works within a small degree of rotation.
The one on my truck is totally operational however not hooked up and I can’t seem to get the right rotation to make it stay on full brakes mode.
When in full mode if your rear brakes are good it they may be too grabby if on smaller tires.
Been meaning to bypass it as with the super heavy tires and wheels, the extra rear braking is great.

 

[Josh577]

GM is cheap. They don't install parts if not needed. Carefully analyze any brake modification.

Side note, man you eastern folks have to deal with a lot of rust. Don't think many here would even consider fixing a vehicle in that condition.

yeah, we are not scared of a challenge ever hear.. lol

 

[fast 99]

Been meaning to bypass it as with the super heavy tires and wheels, the extra rear braking is great.

Can increase rear cylinder bore or use cheap, softer linings to increase rear braking effectiveness.

 

[Grit dog]

Can increase rear cylinder bore or use cheap, softer linings to increase rear braking effectiveness.

My point was, and GMs as well, that the big rear drums on these trucks work TOO well when not loaded down. Hence the additional valve to reduce rear braking bias when empty and light over the rear axle.

 

[fast 99]

Misunderstood, that depends on the application or basically axle weight. Vehicles are built with a lot of compromises, that would be one. Point I was trying to make is braking action can be moved around somewhat.

Most of the trucks I worked on were either tow trucks, concrete cutter vans, plumbing or concrete foundation companies. All were heavily loaded; brake lock up was only dream. In order to slow front pad wear I tried almost everything to increase and equalize brake life, save the customer money and down time.

 

[Grit dog]

Misunderstood, that depends on the application or basically axle weight. Vehicles are built with a lot of compromises, that would be one. Point I was trying to make is braking action can be moved around somewhat.

Most of the trucks I worked on were either tow trucks, concrete cutter vans, plumbing or concrete foundation companies. All were heavily loaded; brake lock up was only dream. In order to slow front pad wear I tried almost everything to increase and equalize brake life, save the customer money and down time.

Well that makes sense. I was speaking to the context of the thread in which I doubt the father son C20 project is being built as a hay hauler or welding rig.