Technical An interesting observation, no rear brakes

I'm having a really hard time believing a 40 year span of brake work and not ever encountering one of these stuck shuttle valves or switch pins. I'm not saying your experience isn't what you say it is, but it must be sheltered or I maybe i must be blessed with these things. The first time I encountered one, well it was a head scratching experience that defies logic. Until someone told me about the possibility then the light bulb moment comes.

You've not had to use the special tools to hold the pin sticking out of the brass block for bleeding either?

They don't always get stuck after a failure, sometimes just pumping the pedal & building pressure in both sides when the light goes out will correct it. Sometimes opening a bleeder on the opposite side and slamming the brake pedal will be enough to move it.

Perhaps its regional and due to climate and corrosion. The ones I've encountered were on vehicles that suffered rusted brake line failure and that valve gets jammed- mostly to the rear circuit. Now I've not seen one that blocks it fully at 100% but certainly up to 90%. It's tell tail sign is found during the start of the bleeding process starting at the rear. There's very very little fluid transfer and sort of just dribbles out of the blocked circuit.

You 40 year guys that haven't ever seen this, pm me your address and ill send you one of these combination valves that's stuck.

 

Never had to remove the little rubber boot?

Never needed the special tool?

 

you got yourself one of them fancy tools , hardly ever ran into problems with a Combo valve not resetting . the later ones often reset just by bleeding the brakes ( pressure centering )

 

31Vicky, the metering valves have to be bypassed to enable low pressure bleeding, and some (maybe most) GM metering valves required a special tool to pull the pin out, as you show, while most Bendix/K/H pins are pushed in to bypass. Some did need the rubber boot to be removed, if memory serves me correctly.
I'll admit most of my experience in the '70s/'90s was other than GM, but I don't ever remember ever reading or hearing about a differential pressure switch that was designed to fully block off any fluid flow. The internal switch movement may interfere with flow, but not prevent it.

 

I had no rear brakes on a 40 coupe. It had a combo valve as 31Vicky has posted. The valve had rust in it, the switch position pin was stuck. I disassembled it, cleaned it up and centered the switch pin with a small tool placed down in the warning switch hole. I "stomped" the brake pedal numerous times trying to get the pin to center, and I think that will work, but I wound up resetting the pin manually.

 

Wolfcreek-steve isn't making it up!

 

Thank you all for weighing in on the brake problem. Tonight I pulled the rear drums and had my daughter push the brake pedal. Both rear wheels cylinders reacted the same way... the front shoe moved outward. I limited it's travel with a clamp on the backing plate so I wouldn't blow the piston out. Once the front facing shoe was constrained, the rear facing shoe would actuate. So, the wheel cylinders aren't frozen. With the drums back on, applying pedal pressure will lock up the wheel but I have no idea of the break away torque. Now, a guy over on Garage Journal says the MC I have is for a disk/disk setup. And the rear brakes should be fed from the front half of the MC. I don't remember where I bought this thing and I can't find any info on it in my paperwork or build book The combination valve came plumbed to the MC and the rear half of the MC feeds the rear brakes. Do I have the incorrect MC?

 

OEm setups are the rear portion of the MC is near the firewall is the front brakes and the front portion is the rears , so the rears actually take up and create hydraulic pressure against the front brake portion of the cylinder as its mechanically connected only for when a system fails by a rod , just hydraulically connected when the system is operating correctly (fluid from the second chamber pushes agianst piston of front chambers back , if your using a Oem type master ( not a wilwood ) you should have it plumbed the Oem way , otherwise your actually taking up the excess space from the pads first then activating the rear brakes after its hydraulicallicly locked and might not be creating enough pressure on the rear portion of the system to apply the shoes tight. but the pedal would be hard or spongy from the calipers movement .

 

as for the wrong MC , its a 1" manual or power it should work , the only thing is if the combi valve is for disc or drum thats the question , as they do have different metering pressures and you cannot change them , you have to change the valve mechanism ( block ) out with the proper unit ( $60 from MBM ) .

 

Hmmm, this is interesting. I was thinking something like that may be happening. I wonder if it's worth re-plumbing the MC that I have?

 

I can check the bore on the MC. As for the combination valve, is there any way of telling what it's designed for by looking at it?

 

I found the vendor where I got my MC and Combination valve. It's an eBay store called Gearhead enterprizes. My order history shows the unit I bought and it's described as a Corvette style 1" bore master with a disk/drum Combination Proportioning/ metering/residual pessure and failure light switch. And it's plumbed with the back half of the valve to the back half of the cylinder. So as far as I can tell, I bought the right thing and installed it the right way.

 

I had a guy who was having basically the same problem with the same MC on a O/T car told him to reverse the master fluid directing and the problem went away . I do not see why companies would do things other than the way the major Oems have done for years other than it makes it easier for them to make parts , as the OEMs have invested time and money to designing systems that work effectively .

 

I agree on doing things they way the OEMs do it. No one else has the engineering horsepower and resources to perfect a design like the big guys.

 

Dual master cylinder port orientation is not "universal", as several OEM master cylinders were plumbed with the primary (closest to the mounting flange) to the rear brakes. The plumbing could even vary on the same vehicle platform, depending on the use; ie, police or normal production. Below is one example of a '70s disc/drum GM master with primary (small chamber) rear brake plumbing.


Most OE, and possibly some aftermarket dual masters are not 50/50 in operation, and correct front/rear plumbing is critical for proper operation. With a 50/50 master, it normally won't matter which axle is connected to the primary.
You're right about the OE factory engineering resources, and is one good reason NOT to use the commonly sold fixed value combination valve on a custom non-factory brake system. The factory didn't use just one combo valve for all their production. You are much better off just using an adjustable proportioning valve with any custom disc/drum or disc/disc system, along with any necessary residual valves, depending on M/C location and drum brake age.

 

There really is no way to place blanket statements on all of this stuff.

 

Just copied this from the ECI brake site:


Hold-Off / Metering Valves are used in the front (disc) system of a disc/drum brake system. They provide a "hold off" function to allow the rear (drum) brakes to actuate first. This function is very important in making the system function in the correct sequence in a rear wheel drive car. The rear brakes are always actuated first. This function is built into most factory type disc/drum combination valves. Make sure you have a metering/hold-off valve in the system either as a stand alone valve or as part of a factory valve.

Proportioning Valves go in the rear brake system (disc or drum) and provide for control of the rate of pressure rise to the rear brakes -- just the rate at which it builds up. Sooner or later the rear brakes see full master cylinder discharge pressure. The purpose of this rate of pressure rise control is to compensate for the reduction of weight on the rear wheels due to forward weight transfer during braking. In short, it eases the application of the rear brakes to help prevent rear wheel lockup. Factory combination valves have these built in -- make sure you know what you're getting, too little rate of rise is as bad as too much. You may not need one of these valves depending on the compatibility of your vehicle's brake system components.

Adjustable Proportioning Valves allow for fine tuning of the rate of pressure rise to the rear brakes if you have a lock up problem. If you have a lockup problem, experiment with the setting of the valve to eliminate lock up for all but all out panic stops. Good luck!

What think ye? If these guys are correct, then this is not a proportioning valve problem. I think I need to revisit the mechanical linkage and be sure I'm getting full travel of the MC piston when the brakes are floored.

 

if its a power system the biggest culprit is not adjusting the rod from the booster to the master when installing them , on manuals I have had several were the guys reused the short power brake rod with the deep bore manual masters . also check your master as some have the deep bore ( manual ) and need the adaptor to take up the space in the bore . for power its suposed to be shallow , manual deep to retain the rod

 

I will check the master cylinder for the deep bore vs. shallow. I don't recall taking the MC off the booster, it came assembled and I assumed it would be correct. I'm thinking its possible that my pedal may be out of travel before my master is.

 

I just worked on the neighbors 95 Blazer last week. The brake line that goes to the rear rusted through. He had it replaced and they couldn't get any fluid to the rear when they tried to bleed the brakes. They changed the master cylinder and still couldn't get fluid to the rear even though it had good pedal height. I had told him about the combination valve blocking the port. He told the shop that was working on it and they told him they had worked on brakes for 20 years and that valve couldn't block the fluid.

He brought it to my house. There was pressure into the combo valve on both master cylinder lines but none out on the rear. I took it apart and got it centered. Fluid would flow through it without touching the pedal. I re-connected the rear line, opened the rear bleeders and let it sit while we had a couple of beers. When fluid came out of the bleeders. I closed them and had him push the pedal down slowly and I opened the bleeders. After bleeding it had good brakes.

 

A quick question... If the combination valve is blocking the port, wouldn't the brake failure indicator light be on? I am going to check the state of that switch on my combo valve. And for you guys who haven't heard of the ports being blocked by the movement of the combination valve, I did read about it last night online. I'm going to see if I can find it and post it up here.

 

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So I see you're blessed too.
The funny thing is the guys with decades of experience think there's nothing left to learn, sooo they throw parts like throwing shit on the wall hoping something sticks. Then they expect payment for their nincompoopery all the while sighting in the messenger when he brings the solution.

 

Lmao -
sure you read it, they can and have read it too, Some have read it several times. Yet they still say it can't happen, won't happen, and scoff at the suggestion and puff up with fluster

 

I too, have got over 40 years in working on and building cars. It's only normal that the longer you're doing this, the higher the probability that you're going to run into something that stumps you! Add to this that right now, I'm working on a system that's part stock, part online hotrod components and part
"I made it myself in my garage!"and when there's a problem, there are lots of new questions to be answered.
I'm always learning, however I will usually defer to manufacturers web sites and specifications as opposed to "I had a buddy whose sister in law had a Chevy that did that".One thing is for sure, nothing can really be fixed or properly designed until you fully understand the principles by which it operates.

 

Didn't even know about combo valves until now. SAE paper on testing combo valves circa 1972. Click on View, right side menu.

http://papers.sae.org/720091/

 

I did brake jobs daily for years and taught others to do them for years and not once in all of my manuals and or collection of auto mechanics text books or in personal experience have I seen the shuttle valve or what ever name you stick on it block off fluid flow to one end of the brakes or the other. This thread is the first reference to something like that I've seen anywhere in over 50 years of working on cars so I'm going to attribute that to half baked theory rather than fact.
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After doing a few minutes research I found this on how things work and it is a pretty solid explanation on how combination valves work an note there is nothing to indicate that they shut off fluid flow in case of a leak in one side.

Metering Valve
The metering valve section of the combination valve is required on cars that have disc brakes on the front wheels and drum brakes on the rear wheels. If you have read How Disc Brakes Works and How Drum Brakes Work, you know that the disc brake pad is normally in contact with the disc, while the drum brake shoes are normally pulled away from the drum. Because of this, the disc brakes are in a position to engage before the drum brakes when you push the brake pedal down.

The metering valve compensates for this, making the drum brakes engage just before the disc brakes. The metering valve does not allow any pressure to the disc brakes until a threshold pressure has been reached. The threshold pressure is low compared to the maximum pressure in the braking system, so the drum brakes just barely engage before the disc brakes kick in.

Having the rear brakes engage before the front brakes provides a lot more stability during braking. Applying the rear brakes first helps keep the car in a straight line, much like the rudder helps a plane fly in straight line.

Pressure Differential Switch
The pressure differential valve is the device that alerts you if you have a leak in one of your brake circuits. The valve contains a specially shaped piston in the middle of a cylinder. Each side of the piston is exposed to the pressure in one of the two brake circuits. As long as the pressure in both circuits is the same, the piston will stay centered in its cylinder. But if one side develops a leak, the pressure will drop in that circuit, forcing the piston off-center. This closes a switch, which turns on a light in the instrument panel of the car. The wires for this switch are visible in the picture above.

Proportioning Valve
The proportioning valve reduces the pressure to the rear brakes. Regardless of what type of brakes a car has, the rear brakes require less force than the front brakes.

The amount of brake force that can be applied to a wheel without locking it depends on the amount of weight on the wheel. More weight means more brake force can be applied. If you have ever slammed on your brakes, you know that an abrupt stop makes your car lean forward. The front gets lower and the back gets higher. This is because a lot of weight is transferred to the front of the car when you stop. Also, most cars have more weight over the front wheels to start with because that is where the engine is located.

If equal braking force were applied at all four wheels during a stop, the rear wheels would lock up before the front wheels. The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force. If the proportioning valve were set to 70 percent and the brake pressure were 1,000 pounds per square inch (psi) for the front brakes, the rear brakes would get 700 psi.
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On the rig in question I'd think the brakes not being adjusted properly have a lot to do with the tires turning while sitting with the brake on on ice or snow. Just as you can often back a car up with the park brake on with poor adjusted brakes and then they drag or hold when you try to go forward.

From my own experience with my 48 if it sat for a while I ended up having to bleed the rear brakes and I attributed that to the cups in the wheel cylinders not maintaining a seal, while sitting for extended periods of time. Drive it every day or several days a week and no problem, let it sit for over a month and there was a problem.

 

Half baked theory ? Hummmmm.
I'm forced to reject your reality by just inserting ALL of the facts.

I'm telling you, as sure as you are reading this under the right conditions that it will block fluid flow. Is it designed to do this? Is that its purpose? Read the patents. What's supposed to happen is a pressure differential will slide the doohickey. Sliding the doohickey does 2 things, 1 is turn on the light, and second is override the metering valve aspect and just let whatever brakes you have left stop you.

Its not supposed to get stuck But it does happen. The conditions that I've see it happen with are thus: rust ruptured rear line- the symptom you see after repair is minimal fluid flow to the rear that defy logic.

Dissect engine mans post and come up with another plausible reason.
( he's got pressure out of the new master into the valve both front and rear, but no pressure out of valve to the rear, pressure to the front only.)Now Unless his methods or description are wrong , there's something stuck in the valve.

It really makes no difference how many times that you haven't seen it, its not some bullshit like the world being flat.

 

I found it! I found it!
Special Purpose Pressure Differential Switch:
There are a number of pressure differential switches that perform a second function. In addition to completing the ground to the red warning light, they also shut off the fluid flow to the rear brakes if a rear hydraulic failure has occurred. The fluid flow to the rear brakes is through the ports in Figure 21. If a rear pressure loss occurs then the pressure differential piston moves toward the rear brake circuit and closes off the fluid passages to the rear brakes as in Figure 22. When dealing with this type of valve you must re-center the valve before the rear brakes can be bled. Bleed the lines at the master cylinder and the fitting on the valve. Open a front bleeder and spike the brake pedal a couple of times taking care not to exceed half pedal travel. This usually will re-center the piston. If it won't re-center, the combination valve will have to be replaced.

 

If the light is connected and works, it should be on. Not many people hook these up in their street rod and mechanics who can't figure out how to get it to turn off will unhook the wire or remove the bulb.

 

I found the same old 2000 site, and most of the info is correct. What doesn't make any sense is why you would want this feature, but more importantly, why you would not want the front system to do the same? It's just not logical. Why would it be?
I think Bill Williams may be confused about the rear proportioning bypass feature when there is a front brake system pressure loss, allowing full master cylinder pressure to the rears, although his pictures do not show this.
I will try to make contact with Bill, but so far no luck. If that changes, I will comment.