Brake System and Combination/Proportioning Valve Diagnosis

[87V30]

Hello,

I have been rebuilding my 87 V30 for several years and recently got it moving under its own power. I am posting as have run into a brake system issue I discovered because of the combination valve/ proportioning valve warning light coming on and immediately coming off at different stages of pedal engagement.

My truck has a remanufactured hydroboost brake booster, remanufactured master cylinder, new stainless steel hard lines, new rubber hoses, fresh calipers, rebuilt drums and new pads/shoes.

I have been moving the truck in an out of the garage and in the driveway for several weeks without issue and a nice pedal feel. This past week I installed the instrument cluster/dashboard and had the warning light illuminate and go off at different points in the pedal throw.

I verified the park brake and metering valve warning switch circuits are correct and not shorted to ground. Further, I was able to have a friend depress the brake pedal while I looked into the metering valve switch port and saw the valve slide to the driver side which tells me there is an issue somewhere with the front braking system.

I reset the proportioning valve to center and used a plastic thread-in tool to bleed the front brake system in hopes of finding a leak point. There was a significant amount of air in the system that just kept coming. I am under the impression there is a leak somewhere in the circuit but I see no puddles or drips at fitting points or changes in master cylinder reservoir fluid levels. I have verified all the fittings are tight and have been watching for leaks. My pedal has remained firm this whole time and is within the specified 3.5” of travel. The rear drums/adjusters have been rechecked and adjusted per the GM light duty truck service manual. On install I did a thorough bench bleed of the master cylinder too.

Am I overlooking something? Is it possible for air to enter the system without an obvious fluid leak or for a failing master cylinder seal to introduce air into only half of the system? Any input is appreciated. Thanks.

I am attempting to attach an image showing the valve movement in this case.

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[Charlie]

 

[smoothandlow84]

Before installing the hydroboost...did you bench bleed the resevoir? I dont know much about the prop valve, but it sounds like one of the front circuits is in fail safe thus preventing one of the front brakes from functioning equal to the other. I am subscribed. ..because i recently converted my 64 front manual drums to vac assist disc brakes. I was informed by a tech at the shop that i purchased my booster from, that I could develop this issue. Mainly it is due to air becoming trapped causing the circuit to fail safe. I too am curious as to what is causing this problem and more importantly how to reset the circuit.


Quick question...did you plumb your front brake lines independent or combine them into one line and tee them under the motor?

 

[Frankenchevy]

He said he bench bled the MC in his post unless you mean something else. The proportioning valve does the job of splitting the front supply to the calipers.

@87V30 , have you tried to vacuum bleed the lines or just the pump and hold method?

 

[75gmck25]

I know you think its probably the front brake system, but I would also carefully check the rear system. I assume you have a FF axle on a V30, so checking the wheel cylinders in the rear is a complicated job that is not done often. When I started getting a warning light on my K25, it turned out to be the rear wheel cylinders leaking. It was an expensive brake job, since they had to pull the axles first, and they also determined the 13" rear drums were worn and needed to be replaced.

Bruce

 

[87V30]

Thanks for the replies.

@smoothandlow84 The lines are independent for the front once they exit the valve like Frankenchevy said. To reset the electrical circuit I am able to both get it to go off with different application of the pedal and physically moving the internal valve back to center.

@Frankenchevy I vacuum bled the brakes initially otherwise I'm not sure I would have ever gotten fluid to the rear cylinders. Since the initial bleed I've been using the pump and hold method but between suspected air/leaks in the front system and air racing past the bleeder screws I've gone back to the vacuum method.

@75gmck25 You make a good point which it did cross my mind that I'm looking in the wrong place but I am trying easier to get to methods first.

Since posting this last night I have rebled the front system and still had the same tripped warning light come on. Beyond the bleeding, I plugged the the front line to caliper ports on the distribution block so no air was at the port locations. When I depressed the pedal the warning light was tripped again. This then leads me to believe the issue is upstream either coming form the master cylinder or combination valve (a new block). I am also placing the front brake circuits under vacuum to leak test them.

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The procedure I followed to bench bleed the MC was place to it leveled in a vice not damaging the body, then filling the reservoir and waiting for fluid to weep out of both ports to avoid damaging dry seals. I then plugged the ports with the thread plastic plugs provided and depressed the plunger not more than 1". Within depressing the plunger a few times it became firm and I could not depress it more than 1/4" and stopped seeing large air bubbles. Next I tilted the MC at a slight incline and repeated depressing the plunger and then put the MC at a slight decline and depressed so more. I returned the MC to level and verified the plunger travel at roughly 1/8". I then bolted the MC to the booster and began the bleeding procedure.

I will try rebleeding the MC although it does not feel spongy, sink to the floor or anything like that. I have also noticed trace brake fluid the warning light switch and plastic brake bleeding valve place holding tool. Not sure if that portion of the valve has fluid in it or maybe a failing o-ring on the valve in the block.

 

[smoothandlow84]

He said he bench bled the MC in his post unless you mean something else. The proportioning valve does the job of splitting the front supply to the calipers.

@87V30 , have you tried to vacuum bleed the lines or just the pump and hold method?

What I meant was...did he plumb each front line seperately off of the prop valve or did he plumb obe line for front brakes at the prop valve then install a tee downstream. Some people only plumb one of the front brakes off the prop valve, while othere use the individual circuits for the front. I personally like the idea of running individual front lines off of the prop valve for safety and failsafe reasons.

I didn't realize that he bench bled the valve prior to install. I was just trying to cover all of the bases and situations in order to detrmine the issue.

 

[Blue Ox]

The procedure I followed to bench bleed the MC was place to it leveled in a vice not damaging the body, then filling the reservoir and waiting for fluid to weep out of both ports to avoid damaging dry seals. I then plugged the ports with the thread plastic plugs provided and depressed the plunger not more than 1". Within depressing the plunger a few times it became firm and I could not depress it more than 1/4" and stopped seeing large air bubbles. Next I tilted the MC at a slight incline and repeated depressing the plunger and then put the MC at a slight decline and depressed so more. I returned the MC to level and verified the plunger travel at roughly 1/8". I then bolted the MC to the booster and began the bleeding procedure.

I'm not saying you did it wrong, but I just did a master on mine. I used tubing to cycle the fluid from the ports back into the reservoir. It was the recommended procedure, and it allowed me to see when the bubbles finally cleared. It took a LOT of cycles before I was able to purge the air. Might be something to revisit.

 

[87V30]

@smoothandlow84 I came across some diagrams while working on this that illustrate the system you describe.

@Blue Ox I saw your reply after I got back to working on the brakes, I rebled my MC with the method I was describing above but the method you used seems to be the most popular method I was just using what was in the supplied kit.

This reply is hopefully to conclude this issue and potentially help somebody in the future with a similar problem:

I did some further testing and appear to have fixed the issues in my brake system. I placed both calipers, hoses, and brake line assemblies under vacuum and they both held 23inHg for at least 40 minutes each. Without a leak present after the outlets on the proportioning valve there had to be an issue somewhere from the MC to the proportioning valve.

The master cylinder was previously bench bled on installation but as some of you pointed out there was the possibility I did not get all the air out. The proportioning valve also is not original equipment: it is an imported part meant to replace the AC DELCO 172-1353/CPP PV-2 disc drum valve. Trace amounts of brake fluid were noted on the valve holding tool and on the warning light switch (I suspected there was the possibility of a failing o-ring/internal leak at this point).

I disconnected the master cylinder and plugged its ports in preparation to bench bleed it again. While the system was opened this was also a good time to pull the proportioning valve that came on the truck out of storage and clean it up. I swapped out the imported proportioning valve with the proportioning valve that came off of the truck. I reconnected all of the lines, then bled the MC then installed the MC. I the proceeded to do a gravity bleed and vacuum bleed at the four corners. Did some late night tests and no warning light came on. This morning I verified the warning light comes on when simulating both front and rear blowout/failure situations.

Thanks for the input as it helped the process. Out of curiosity from your experience how long does it take you to bleed a brake system that has been opened/had a line replaced/newly installed on these trucks?

 

[78C10BigTen]

Ive always been told when bleeding to start with the wheel farthest from the mc. Just thought id throw this in.

 

[coachdtierney54]

Not to steal anyone's thunder, but I also recently replaced my MC on my 85 C30. I'm also leaking fluid from somewhere. I bled the brakes starting from the furthest from the MC. No noticable indications of leaks, but I keep having to add fluid.

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[78C10BigTen]

Not to steal anyone's thunder, but I also recently replaced my MC on my 85 C30. I'm also leaking fluid from somewhere. I bled the brakes starting from the furthest from the MC. No noticable indications of leaks, but I keep having to add fluid.

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Seaping at the wheel cylinders?

 

[coachdtierney54]

I failed to mention that I have never had to add fluid to the compartment closest to the cab of the truck. Am I correct in thinking that it is a front brake issue?

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[coachdtierney54]

I also did a complete fluid change. There is an electric brake assist on the truck. Could that be the issue?

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[Blue Ox]

An "electric" brake assist?

Are you talking about the hydroboost? No, that has nothing to do with losing brake fluid. The smaller reservoir in the master is for the rears. You'll need to examine the whole system all the way to the wheel cylinders and look for a leak.