brake proportioning valve

[dooguy]

get a brake lite when connecting wire on proportioning valve thats connected to brake booster on a 71 k20. Thinking on replacing. I believe I have read that parking brake switch can also cause this. Anyone else seen this?

Thanks

[Sheepdip]

Originally Posted by dooguy

get a brake lite when connecting wire on proportioning valve thats connected to brake booster on a 71 k20. Thinking on replacing. I believe I have read that parking brake switch can also cause this. Anyone else seen this?

Thanks

New, existing need more info. If new valve may not be centered if existing could be a failing MC or air.

[old51sedan]

Originally Posted by dooguy

get a brake lite when connecting wire on proportioning valve thats connected to brake booster on a 71 k20. Thinking on replacing. I believe I have read that parking brake switch can also cause this. Anyone else seen this?

Thanks

Interesting, when I installed the new brake proportioning valve and plugged that wire in I started having a problem with my LR tail light turn signal no working. It has to be a ground somewhere........

[Stocker]

Originally Posted by dooguy

I believe I have read that parking brake switch can also cause this.

Parking brake switch? On a '71?

[dooguy]

Originally Posted by Sheepdip

New, existing need more info. If new valve may not be centered if existing could be a failing MC or air.

existing. maybe air since installed existing cab from another chassis

[Stocker]

If everything appears OK, maybe the pin in the prop. valve just is not centered? Never had to do this myself, but supposedly you can press really hard on the brake pedal and the pin will be forced to the center. May be worth a try.

[dmjlambert]

If you have a stock proportioning valve I recommend don't replace it with new, and get it rebuilt instead. It costs more $$$, but works well and is engineered for your truck and not just some generic leaky part. I recommend the folks I used: White Post Restorations https://whitepost.com/brake-sleeving...ding-services/
But before buying or restoring, as Sheepdip says it could be another thing causing the problem, such as master cylinder or air the system.

When I bleed my brakes, the light comes on when one of the front bleeders is open, and the light comes on when one of the rear bleeders is open, this tests the shuttle valve and tests proper operation. If the light stays on, you just stomp a couple times on the brake after closing the bleeders and it will re-center. I would not say very hard is necessary, perhaps hard enough to crack a hard pecan or squash a roach.

If parking brake pedal has a switch on it, it would result in the light coming on before you even try to connect the proportioning valve wire. Adding a parking brake engaged warning light switch is a non-factory home-made addition, so you would probably know if you have it or if it's a new-to-you truck, look up under the dash at the parking brake pedal assembly.

[dooguy]

good to know about proportion valve.

Thanks

[RichardJ]

>>When I bleed my brakes, the light comes on when one of the front bleeders is open, and the light comes on when one of the rear bleeders is open, this tests the shuttle valve and tests proper operation.<<

OK, you have a good understanding that pressure pushes the switch away from the END with high pressure ( bleeder closed) toward the END with low pressure (bleeder open).

Simply close the open bleeder.
Then, open a bleeder on the other END.
Then, push the brake pedal slowly until the light goes out.
Close the open bleeder.

You are using the pressure to slowly move the switch back to the middle position.

By END, I mean front or rear. It doesn't mater if you open left or right side. Both sides will have the same pressure as that side of the Pressure Differential Switch. That's the middle portion of the Combination Valve.

Save the stomping for the cockroaches.

[dmjlambert]

Save the stomping for the cockroaches, that's funny right there!
The 71 and 72 service manuals have the proportioning valve light switch testing procedures. There is mention there about to reset switch, apply heavy pedal force. But the idea of opening the opposite circuit's bleeder and pressing the pedal does sound better, especially if you have a helper to close the bleeder before letting up on the pedal or if you're using a one-man bleeder jar/method.

[RichardJ]

The little piston that is inside the switch will stop moving as soon as you take pressure off the pedal. After you close the bleeder, The pressure on both sides of the switch piston will always be equal and the piston will stay where you left it

[dooguy]

Originally Posted by RichardJ

>>When I bleed my brakes, the light comes on when one of the front bleeders is open, and the light comes on when one of the rear bleeders is open, this tests the shuttle valve and tests proper operation.<<

OK, you have a good understanding that pressure pushes the switch away from the END with high pressure ( bleeder closed) toward the END with low pressure (bleeder open).

Simply close the open bleeder.
Then, open a bleeder on the other END.
Then, push the brake pedal slowly until the light goes out.
Close the open bleeder.

You are using the pressure to slowly move the switch back to the middle position.

By END, I mean front or rear. It doesn't mater if you open left or right side. Both sides will have the same pressure as that side of the Pressure Differential Switch. That's the middle portion of the Combination Valve.

Save the stomping for the cockroaches.

when you speak of the bleeder, are you talking about the bleeder on valve or bleeder at wheel?

[RichardJ]

Brake systems are bled at the drum wheel cylinder or at the caliper, unless you're using a "reverse flow pressure bleeder".

[dooguy]

not knowing any better, I believe my proportioning valve has been stuck for some time after receiving from a shop who I'm thinking did not how to bleed brakes properly.
when i stomp on brake pedal on gravel my rear brakes will lock. front brakes do not.
so to reset switch do i need the tool that goes in where brake lite switch attaches or screws in once i get shuttle valve back to center and then rebleed brakes starting farthest in back and then progressing forward.

My understanding is if switch is not centered and secured in the groove you won't be able to get brake fluid to each wheel when bleeding

sorry if this has been discussed before

[Rust_never_sleeps]

Probably worth visually verifying that the shuttle is offcenter first. Assuming that:
The shuttle valve will move towards low pressure, so whichever way it has shifted, opening any bleed valve in the opposite circuit will move it towards the open side.
As suggested, putting the switch in will let you know when it's back in the center, because the light will go out.

[Rust_never_sleeps]

The tool doesn't move the shuttle valve, it just holds it in place once you get it centered again so it doesn't move again when bleeding

[dooguy]

can I remove switch with brake system pressurized without losing any fluid

[dooguy]

since it appears my front brakes are not engaging, I can/should open bleeder on front brake and push brake pedal to move valve back to center.
could I possibly move valve back manually too by removing switch.

And if I am successful at centering valve, I probably need to rebleed brake system, but I will need a tool or something to hold valve in center while bleeding?

[Rust_never_sleeps]

You can remove the switch without leaking anything.
If you crack a wheel bleeder just like when you were bleeding(tube into bottle, etc), you won't need to rebleed the system. If you still want to, insert the pin to keep it centered.
The shuttle valve will stop moving as soon as you stop moving the pedal. Hold it there and close the bleeder.

[RichardJ]

That junk piece of plastic or TOOL as some call it shouldn't be used with manual bleeding.
When you push on the brake pedal or stomp as some of you think you should do, can generate a considerable amount of pressure (psi). Depending on hard you push, you can easily have 200-300 psi on the end with the closed bleeder, The open bleeder end may have 5-10 psi. When the switch piston moves it will easily break off the nipple on the plastic "tool".
When you use a pressure bleeder, you're using maybe 15 psi of air pressure.
Brake systems can generate well over 1000 psi with heavy braking.

Roadkill Garage clowns, David Freiburger and Steve Dulcich, are pretty knowledgably in some things, but at other times behave like ignorant, hillbilly, shade tree morons.

I watched them manually bleed brakes with that plastic tool on one of their shows.
They broke the nipple off the tool as expected. They plucked the broken piece out with tweezers and centered the switch piston with a pick.
Their brakes worked, but I would have thought they would know better.

[dooguy]

I am a little cornfused. I do need to have valve centered to bleed brakes?
I am having little success with removing brake switch from valve body, probably 50+ years since removed.

Or can I just bleed front brakes(these being the ones that seem not to operate)
and this should move valve back to center.

I was hoping to remove switch and then use a skinny probe and move it back to center

once centered and held in place with something stronger than plastic then I should be able to bleed brakes correctly?

If I understand this, you will not be able to bleed brakes properly unless valve stays centerd

[RichardJ]

How about starting from the beginning.

You have disc/drum brakes? >>existing cab from another chassis<< What do you have?

>>after receiving from a shop who I'm thinking did not how to bleed brakes properly.<< What is the story on that?

[dooguy]

Originally Posted by RichardJ

How about starting from the beginning.

You have disc/drum brakes? >>existing cab from another chassis<< What do you have?

>>after receiving from a shop who I'm thinking did not how to bleed brakes properly.<< What is the story on that?

yes disc/drum brakes. 72 cab on 71 k20 chassis

brakes worked fine when on 2wd chassis

shop that did body transfer did brake setup/connection

brake wire not hooked to switch on proportional valve when received from shop

when connecting I get a brake lite which after discussing this on this site I decided to check brake actuation on gravel road.

look to have no front brake actuation.

So I am assuming shuttle valve is diverting all braking to rear because it is stuck

can this shuttle valve be recentered by opening bleed valve on front caliper

[dmjlambert]

Originally Posted by dooguy

I am a little cornfused. I do need to have valve centered to bleed brakes?
I am having little success with removing brake switch from valve body, probably 50+ years since removed.

Or can I just bleed front brakes(these being the ones that seem not to operate)
and this should move valve back to center.

I was hoping to remove switch and then use a skinny probe and move it back to center

once centered and held in place with something stronger than plastic then I should be able to bleed brakes correctly?

If I understand this, you will not be able to bleed brakes properly unless valve stays centered

Not being able to bleed brakes properly unless valve stays centered is a very popular myth. Just not true. The idea of the valve is this: when you have a leak in the tubing or caliper that leads to the front brakes, the valve mostly closes off the fluid going to those leaky front brakes and it shorts the electric terminal to ground which lights your warning light. If instead it was the tubing that goes to the rear or rear wheel cylinder, the valve mostly closes off the fluid going to the leaky rear brakes and it shorts the electric terminal to ground which lights your warning light. When you have no leaks and a properly working valve, it will be somewhat centered and the electric contact will not connect to ground inside the valve, and front and rear brakes will work fine. So if you observe the light when bleeding your brakes, you are checking the functionality of that valve and warning light switch. It should not jam itself into a locked position, unless you are stomping on the brakes during bleeding.

I don't think there is any way to tell for sure whether the valve is stuck forward or stuck rearward, in either case the light will light. If you have the key on, and you open a rear brake bleeder (which creates leaky brakes) and you press the pedal (you don't need to stomp, just press like you're braking) and the light goes out for a little bit, but comes back on, that means the valve is not stuck. It is just shifted from the back to the front. If it doesn't go out at all, it could mean the valve is already switched to alarm about the rear brakes. So close the rear brake bleeder and open a front bleeder and repeat the procedure. If the light never goes out, then it could be the valve is stuck and needs rebuilding or replacement. Moving the valve by opening a bleeder and observing the light going off is the procedure that RichardJ was talking about earlier in the thread. Or, according to the service manual, if you have all the bleeders closed (and you also have no leaks) and you apply heavy pedal force, it will reset the valve/switch and turn out the light. I have done that, and observed that method definitely works. But it does require the valve to be in good condition and not sticking due to excessive wear and rust inside. I recommend trying RichardJ's method and see if that valve will move and the light go out from opening a bleeder in the front, press the brake slowly and if that doesn't turn out the light, close the bleeder and open a back bleeder and repeat.

If you have a cast iron valve, that is worth sending off for rebuilding in my opinion.

It is normal if the back brakes lock before the front on a C10 that does not have 1000 lbs of stuff in the bed (or 1500 lbs for a K20). The brakes would have been engineered to stop the truck with balance and efficiency when used as a hauling truck at capacity. If you want to know if the front brakes are working, go for a mile or two while stopping and going over and over, and when you get back to the house use one of those handheld laser thermometers to measure the temperature of your discs and drums. If they are same temperature as the weather is outside, the brake isn't engaging at all. The whole purpose of the brakes is to convert the kinetic energy of the moving truck into heat energy, it's what the brakes do.

[dooguy]

thanks for all the info. I haven't been able to gleen anything locally. my valve body is cast iron. Classic chevrolet mwebsite shows a valve that appears to be cast iron $85.00. local mechanic told me he just replaces them if not working but i think the real life experience I'm reading here is probably better advice