Proportioning Valve Keeps Tripping?

[KyleSeal]

1967 C10 - Disc conversion (power 8" booster) - master cylinder is 1" bore, and the proportioning valve was advertised when bought in the kit as a disc/drum setup.

I am having an issue with my proportioning valve.
When I go to bleed the brakes, while using the proportioning valve tool to center the shuttle valve, everything bleeds fine, no leaks and good strong flow. No air coming out anymore, I did plenty of extra pedal strokes to ensure this and had no air that I could see come out.
But when I put the brake switch back in, bleeders all closed, and press the pedal down half way, the red brake light illuminates. I have pulled the switch and confirmed the shuttle valve closes the rear brakes. Recenter, does it again.
I confirmed the rear brake shoes are at their proper adjustment with a brake drum/shoe caliper tool to get the measurement even.
Even after adjusting and all this bleeding, it seems like its either seeing a fault at the rear.

So my question essentially is how do I move forward from here?
From the factory, the front brakes are fed by 1 line off the front of this proportioning valve down to a T fitting split to both sides, and obviously rear is one that goes to a T on the differential.
Someone did say my front feed being at the top fitting (pictured) is an issue, I don't see this as an issue being this should be able to balance it, am I incorrect?

[72SB]

The position of front line is not the problem.

Your PV is sensing a change which triggers the switch. Whatever direction it moves, that part (front or back) has a leak you need to find

...or the PV is bad which is rare but given source of replacement parts used in these kits, way more possible now.

Power brake disc typically use a 1 1/8" bore MC but that would not affect what PV is doing. larger bore just means less pedal stroke as the booster assist does not need the stroke a manual setup needs.

[Dashman]

In my opinion the front brake line coming off the combination valve upwards shouldn't matter because it was designed that way for two front brake lines. However, typically when using a single line with a tee, others (factory included?) used the bottom port and plugged the top. I don't think it will matter.

Someone else, on this forum, stated that their switch was too sensitive. Are you seeing a lot of movement of the shuttle valve? Having removed the switch, how much movement can it have before the light turns on?

How do you know that the proportioning valve isn't stuck in the open position? It's function is too reduce the pressure to the rear, right? What would the shuttle valve do, if the proportioning valve wasn't doing its function? I think it would slide rearward a bit. You could replace just the proportioning valve, and see if it makes a difference. Then you could disassemble the first proportioning valve to see if there was a problem.

[RichardJ]

>>proper adjustment with a brake drum/shoe caliper tool <<

If you are talking about a "resetting tool" , the shoes are probably still too far away from the drums.
You should adjust until the shoes drag and then step on the brake pedal to center the shoes. You have to readjust tighter, press on the brake and check for drag again. The finish adjustment should allow you to hear some drag, but not necessarily feel any resistance when you turn the drum.

[kwmech]

Which mater cylinder do you have on there? Lift the cap off and take a pic.

[KyleSeal]

Originally Posted by 72SB

The position of front line is not the problem.

Your PV is sensing a change which triggers the switch. Whatever direction it moves, that part (front or back) has a leak you need to find

...or the PV is bad which is rare but given source of replacement parts used in these kits, way more possible now.

Power brake disc typically use a 1 1/8" bore MC but that would not affect what PV is doing. larger bore just means less pedal stroke as the booster assist does not need the stroke a manual setup needs.

I have checked all fittings - no leaks. Verified visually and with a clean rag to see if theres even the smallest seepage. I am leaning towards a bad PV.

Originally Posted by Dashman

In my opinion the front brake line coming off the combination valve upwards shouldn't matter because it was designed that way for two front brake lines. However, typically when using a single line with a tee, others (factory included?) used the bottom port and plugged the top. I don't think it will matter.

Someone else, on this forum, stated that their switch was too sensitive. Are you seeing a lot of movement of the shuttle valve? Having removed the switch, how much movement can it have before the light turns on?

How do you know that the proportioning valve isn't stuck in the open position? It's function is too reduce the pressure to the rear, right? What would the shuttle valve do, if the proportioning valve wasn't doing its function? I think it would slide rearward a bit. You could replace just the proportioning valve, and see if it makes a difference. Then you could disassemble the first proportioning valve to see if there was a problem.

Ill need to visually see how much it moves, it moves enough to trigger but I can still manually recenter with a pick, moving the shuttle back to neutral. Its somewhat "U" shaped, and I can grab the edge of the U and pull it back, if that makes any sense.

Originally Posted by RichardJ

>>proper adjustment with a brake drum/shoe caliper tool <<

If you are talking about a "resetting tool" , the shoes are probably still too far away from the drums.
You should adjust until the shoes drag and then step on the brake pedal to center the shoes. You have to readjust tighter, press on the brake and check for drag again. The finish adjustment should allow you to hear some drag, but not necessarily feel any resistance when you turn the drum.

The tool I am refering to is a tool made by Blue Point, its a giant caliper that you set inside the drum to set the diameter, and then hold over the shoes and adjust until they lightly touch. I dont believe the rear shoes are an issue; theyre adjusted to the proper drag currently and the light is still persisting after resetting the PV and verifying it is in neutral and the light is off until the pedal is pressed.

[MikeB]

My almost-new switch all of a sudden got very sensitive, even when not applying the brakes. I pulled it out and checked it on the bench with an ohmmeter. Turns out that just a very small bit of upward movement causes the switch to close. I mean we're talking .050" if that much.

Ended up using the later model switch that came with my aftermarket combo valve. Unfortunately, the wire connector is different, but that's no big deal -- just another PITA to deal with.

This is probably not your problem, since your switch closes only when you step on the brakes. But I just wanted to get this info out there -- the light flickering or being on all the time could indicate a bad ChiCom switch.

On another note, that's a dual 8" booster, right?

[Rust_never_sleeps]

Maybe try adjusting the backs to be stupidly tight and see if that makes a difference.
IF you still have the differential pressure valve moving, then there's air or fluid loss somewhere causing a....pressure difference

[KyleSeal]

Originally Posted by MikeB

My almost-new switch all of a sudden got very sensitive, even when not applying the brakes. I pulled it out and checked it on the bench with an ohmmeter. Turns out that just a very small bit of upward movement causes the switch to close. I mean we're talking .050" if that much.

Ended up using the later model switch that came with my aftermarket combo valve. Unfortunately, the wire connector is different, but that's no big deal -- just another PITA to deal with.

This is probably not your problem, since your switch closes only when you step on the brakes. But I just wanted to get this info out there -- the light flickering or being on all the time could indicate a bad ChiCom switch.

On another note, that's a dual 8" booster, right?

Yep - 8" booster. I did flush mount to the firewall, which I was thinking may be a problem, but judging by what others have told me I am thinking I have some air really stuck. Im going to try a heavy hit to the pedal with the bleeders open and see what it gets me. Hopefully not a mess to clean up!

Originally Posted by Rust_never_sleeps

Maybe try adjusting the backs to be stupidly tight and see if that makes a difference.
IF you still have the differential pressure valve moving, then there's air or fluid loss somewhere causing a....pressure difference

I actually did adjust them to the point where the wheel wont turn. Still the same. I am thinking I have a huge bubble somewhere and its possibly in the rear circuit at the prop or in the prop valve.

[Rust_never_sleeps]

I actually did adjust them to the point where the wheel wont turn. Still the same. I am thinking I have a huge bubble somewhere and its possibly in the rear circuit at the prop or in the prop valve.

Gotcha. I had better luck syringe bleeding the MC/combo valve than bench bleeding, or at least felt more certain about it.
Bought a pack of these off the River :shrug:

[KyleSeal]

Originally Posted by Rust_never_sleeps

Gotcha. I had better luck syringe bleeding the MC/combo valve than bench bleeding, or at least felt more certain about it.
Bought a pack of these off the River :shrug:

Ive thought of this as well when I was bench bleeding the MC. That may be on the list after some abrupt pressure bleeding.

[72c20customcamper]

When the light goes on do the rear brakes still actuate? The proportion valve when tripped will stop the fluid from going to that circuit. I would let the light come on and try to bleed the rear brakes no fluid the circuit is tripped if you get fluid then it may be the switch .

[MikeB]

Originally Posted by KyleSeal

YI am thinking I have a huge bubble somewhere and its possibly in the rear circuit at the prop or in the prop valve.

How does the pedal feel? Can't imagine there's air in the lines after all the bleeding you've done.

Have you bled brakes in the past? Are you shutting the bleeder valve before your helper lets off the pedal? And/or submerging the bleeder hose in fluid so it can't suck in air?

[KyleSeal]

Originally Posted by MikeB

How does the pedal feel? Can't imagine there's air in the lines after all the bleeding you've done.

Have you bled brakes in the past? Are you shutting the bleeder valve before your helper lets off the pedal? And/or submerging the bleeder hose in fluid so it can't suck in air?

Ive done quite a bit of bleeding in the past. I use the one way valve "Speed Bleeders" on nearly everything, so I dont have to on/off the bleeder. I do keep it submerged though. If all else fails, I will borrow a pressure bleeder and force the system to give me an answer!

As far as pedal feel, it is soft sometimes, and sometimes when bleeding it is hard. Counterintuitive.

[71CHEVYSHORTBED402]

Old school reputable mech. says old CPV vales rarely fail.

[KyleSeal]

The plan as of now;
1. I purchased a pressure bleeder, should be here Saturday. I will force all the air and fluid out with it if theres any.
If no avail;
2. I will replace the master and proportioning valve with a 1-1/8" master kit.
If no avail;
3. I will start going to church

Does anyone think the dual-diaphragm 8" booster with a 1" bore master is an issue? I am not thinking it is as I dont know why one would be sold if that was the issue, but now I am possibly beginning to second guess all components.

[72SB]

I did my 72 manual disc/drum to power using the CPP booster/mc/pv. Rears went to rear of PV, front to front of PV. (pic)

O{, I think your issue is the PV or the shuttle switch in it.

OP, power front disc brakes use a 1 1/8" MC. Manual drum uses 1" MC. Either should "work" but with the 1 1/8 MC the pedal stroke will be shorter which with the booster is all that is needed.

[MikeB]

As for bleeding an M/C, I do it on the truck using these hard lines. Surprisingly, not a lot of fluid leaks out when you remove them after bleeding. Sure beats doing it using those M. Mouse kits that come with some M/Cs.

[Rust_never_sleeps]

Originally Posted by MikeB

As for bleeding an M/C, I do it on the truck using these hard lines. Surprisingly, not a lot of fluid leaks out when you remove them after bleeding. Sure beats doing it using those M. Mouse kits that come with some M/Cs.

I rigged up one of these from the old lines(after I lost the plastic pieces that came with the MC). I prolly would have never bothered with syringe bleeding had I known at the time that thrashing on the brakes creates thousands of tiny bubbles through cavitation, as does applying stupid heavy vacuum

[Dashman]

I like Mike B's master cylinder bleeder lines. I was thinking about a tee off the proportioning valve port with a bleeder screw, or straight to a bleeder screw with an adapter (if manufactured), or a tube nut with hard line and vinyl hose. Isn't there a brake line coupler on the frame rail? Maybe you could bleed from there also. I think you're right, it's more likely that you have an air bubble near the start of the rear brake line. Good luck.

[KyleSeal]

This weekend I tried the power bleeder. No dice.

I am beginning to believe I am pulling air, but not leaking fluid. I have heard numerous times now about wheel cylinders bringing mystery air in. These wheel cylinders are probably 8 years old, never used. That being said, I never had fluid in the system to test them. I am going to replace them and recheck.

Everytime I go to bleed the rears, I get micro bubbles and sometimes very small ones, but regardless I get a stream every time I open the bleeder. I ran nearly a gallon of fluid through them and still had the same situation.

I also rebled the master cylinder to ensure no air is trapped in it, and it bled clean.

[54blackhornet]

Kyle I applaud your persistence ! Keep at it ! Lots of people will learn from your experience..Jack

[MikeB]

Originally Posted by KyleSeal

I am beginning to believe I am pulling air, but not leaking fluid. I have heard numerous times now about wheel cylinders bringing mystery air in.

If air is somehow creeping into the system (which I doubt), that means you would have a fluid leak. Have you seen signs of that anywhere?

FYI, if you are pulling fluid trough the bleeder valves using a vacuum pump like a MityVac, you will almost always see bubbles from air creeping in between the hose and bleeder valve.

[dmjlambert]

What are the full specs for the kit, or webpage where it is sold?

[72SB]

It is not uncommon to pull air from bleeder nipples being loosened to bleed at the threads. A thin coat of silicone on the bleeder nipple threads prevents this. Do not use Teflon tape.