The Stovebolt Site Forums The Shop Area Driveline Dual circuit brake system parts diagrams

As I am in the process of installing a new dual circuit system on my '51 3600 (drum/drum), I've been reading through the multitude of threads about what goes where. I've also read several articles in old copies of Vintage Truck magazine. A Google search led me to this site which has some really nice, simple pictures of the parts needed for the different set ups: http://mbmbrakes.com/typical-brake-system-configurations/

I thought I'd share it here to help others who might be in the same boat.

Thanks Brian for the link! I do have a question. The company recommends a 2 lb. residual valve for a master cylinder configuration that is under the floor. Why doesn’t our original system have these? Are they built into the master?

Phil, I can't answer your question as to how the original master cylinder was built. I'm learning all of this stuff for the first time myself. I'll take a look into the m.c. that I took out of my truck and see if it perhaps has a built in check valve of some sort.

The recommendation for a 2 lb. residual valve, though, is for disc brake applications. It certainly wouldn't have been part of our original systems.

The one question that I still have regards the adjustable 10 lb. residual valve for the rear drum brakes that is shown in the drum/drum system. In other diagrams showing the plumbing to the rear drum brakes there is an adjustable valve installed separately from the 10 lb. valve, but nothing said about what that valve does or how it's adjusted. I understand that you want the rear brakes to come on a little earlier than the fronts. From the threads I've read, that action apparently happens inside the master cylinder itself. So what is the adjustable valve for?

I believe the original master cylinders use an internal 10 lb residual valve. I put a proportioning in the rear circuit of my 1 ton because the brakes were too powerful since I don’t carry loads. On slippery surfaces they tended to lock up.

Speedway motors has some great Tech stuff on brakes, this is just one of their layouts. https://www.speedwaymotors.com/the-toolbox/braking-components-selection-and-design/28678

The valve you are talking about i believe is a proportioning valve. we used them in dirt track cars to get the front to back braking to be either more or less pressure in either direction. In the case of a truck, if you have no load, you could use it to reduce the pressure to the back so they dont lock up, and have more pressure if its loaded. look at them as if its a trailer, because they use the same kind of system on trailer brakes.

Both GMC, Toyota @nd probably others have a factory proportioning valve hooked to the rear axle with a rod that adjusts with the load with a connection rod. It should be easy to hook one up oneself to one's own suspension.

Ed

As HRL says, reading is fundamental! I did not see that the diagram I was referring to (last one) was for disk/disk w/master under floor. I thought it was for drum/drum w/master under the floor in which they don’t have a diagram for. My bad, but I am learning allot about brake systems!

The references to "m/c under the floor" or "m/c on the firewall" is misleading. I assume they are talking about lowered street rods. The important thing is this:

In my case the m/c is under the floor. But since it's a truck at close to normal ride height, the m/c is above the calipers and thus no RPV is required.

I have Camaro disk brakes, Camaro rear drum brakes and Camaro m/c with the combination valve. There was no rear proportioning valve in the car and one is not needed tor the truck. Camaros were front heavy, too.

In the diagram for a drum/drum system (in my orginal post) there is a "metering valve" in the line to the front brakes. The description says this is to prevent "nose dive". I found this one https://www.speedwaymotors.com/Brake-Metering-Hold-Off-Valve,8904.html?sku=91031363. Speedway says this is the valve that allows the rear brakes to set up a little sooner than the front brakes. Does anyone have one of these installed in their truck? Is it really necessary?

Edit: I asked the question too soon. The answer was right in front of me thanks to the all-powerful Google. The Summit Racing web site states, "A brake metering valve is used to Balance the Braking Forces. It is only found on vehicles with front disc and rear drum brakes. The metering valve holds back fluid to the front brakes until enough pressure is created to overcome the return springs inside the rear drums. It is also known as a “metering block.”

The metering valve is plumbed into the system immediately after the Master Cylinder. In many vehicles, the metering valve is part of the Combination Valve."

I think I'll install my brake system without this valve. After I've been able to drive the truck around a bit I can then decide if I need to add one or not.

The valve you are talking about i believe is a proportioning valve. we used them in dirt track cars to get the front to back braking to be either more or less pressure in either direction. In the case of a truck, if you have no load, you could use it to reduce the pressure to the back so they dont lock up, and have more pressure if its loaded. look at them as if its a trailer, because they use the same kind of system on trailer brakes.[/quote]

Demo Man, thanks for that anology. Thinking about it in terms of a trailer brake makes a lot of sense. So I'm looking at one of the compact adjustable proportioning valves available since my truck typically won't be hauling a load, and, my bed is all aluminum (pretty light). I can see the rears locking up pretty easy if I don't cut the pressure down.

The original manual style disc/drum master cylinders had a residual pressure valve for the rear brakes because the drum brakes need a little residual line pressure to keep the shoes close to the drum after you get off the brake pedal. So if you took one of those original master cylinders apart, you could remove the brass seat for the brake line at the master, and then would generally find a little rubber "duck bill" residual valve behind it. The dual masters you get today generally don't have the residual valve in them any longer. So installing an inline 10lb valve for the rear drum brakes is one way to keep the line pressure up so the shoes don't overly contract.

The proportioning valve is a little different in that too much pressure makes the shoes hit the drum too hard. So you can dial in a little less pressure to soften the hit so to speak. Really what you are doing is adjusting for the timing of the brakes to apply, and the amount of force. You want them to apply at the right time and with enough force to be effective but not lock up and become ineffective.