The Stovebolt Site Forums The Shop Area Driveline 1957 Chevy Pickup - Dual Master Cylinder Issue

Hi all!

I've had my dual master cylinder mounted for a few months now and finally got ready to go buy brake lines, when I realized I had a problem! My dual master cylinder, that I bought to replace the original, has 4 ports!

I bought this one, apparently by accident: http://www.americanclassic.com/images/BR0129.jpg

It says it's a "1936-1991 Master cylinder, dual reservoir, 1 inch bore, two 3/8 inch ports on each side (total of 4 ports), has deep bore with metal insert for both manual and power applications".

My truck has 4 drum brakes and no prop valve. Am I out of luck? I was considering running a line out of each port, directly to each wheel because I've read that drum cylinder's are metered at the port and don't use a prop valve but I have no idea about brake systems and why that would / wouldn't work.

I've goggled a bunch of various terms but can't find a solution. I'd really prefer not to have to order another one if possible. Any ideas?

That cylinder is for multi use. Just plug the ones on the side you do not use.

Hy VanattiWorkshop, Don't panic, plug the two ports in the master cylinder which are more difficult to reach and plumb to in the truck. Now you can plumb the truck using the primary (the port closest to the pedal) to plumb to front brakes, and the secondary port to plumb the rear system. You can follow the factory plumbing, other than not connecting the front and rear systems. You should not need a proportioning valve for an all drum system. You will have to find out if your master cylinder was designed for drum, disc, or a disc/drum system, if it was for a disc or a disc/drum system you will have to install 10 p.s.i. residual pressure valves in the ports designed for disc brake use, hope that helps.

Unfortunately, I've read the manual and the website I bought it from, and can't seem to find if it's drum only or disc/drum. Still, that helps so much that I can just plug the unused ports.

Out of curiosity, what would happen if I ran a line from each port to each wheel? Uneven braking I'm guessing?

That's fantastic though, I can still go buy my lines. Thanks guys!

So how does one plug a port?

If you hooked up all four in such a way that one side was on one chamber and the other side was on the other, it would be really bad in a failure mode.

Hy Guys, in theory you could indeed use all four ports on the master cylinder, you would just need to use the two primary ports for the front brakes, and the two secondary ports for the rear brakes. You would have to run more tubing, and add another rear flex line if you did do it that way, sounds like a lot of extra work to me. As to pluging the ports, you just need to use an inverted flare plug of the correct size, Weatherhead p/n 131X?, you may have to purchase port adapters to convert the ports to a standard size because the modern master cylinders have all kinds of different size tube nuts goin into them, hope that helps.

I guess you will want to make sure that there are residual valves in your system. I don't know if they would have provided them inside your master.

I think I'll go with the plug route since I already have 3 brake hoses and don't want to order another. I'll probably plug the left front and left rear since those are tucked on the inside of the frame and harder to get to.

Still not sure if I need prop / residual valves so maybe I'll just have to find out the hard way.

Hy VanattiWorkshop, I sent an "E" mail to American Classic, trying to find out what the original application for the master cylinder was full disc, disc/drum, or full drum, I was answered by a budding politician, they said " Trick question. If you plumb it then yes it works for all". So I sent him another "E" mail asking them if their master cylinder p/n BRO129 had built in residual pressure valves. Their answer was "Another trick question. None of them do to my knowledge. You only need them really if you are sighting the master cylinder low under the floor and the fluid will run back. We sell a 2 and a 10 valve for that." So from that I would assume (and yes I know what we do when we do that) that BRO129 would require a 10 p.s.i. residual valve in the front and rear brake circuits. I was extremely disappointed with their response, drum brake master cylinders have residual pressure valves built into them, regardless of where on the vehicle the master cylinder is mounted. Hope that helps

I just purchased one from ebay for my 56 3100 chevy. It looks identical to that picture here. My came with two plugs. My dirrections says add your brake lines, you may use either side of the master cylinder, plug off the other side with the enclosed plugs. Oh the only difference my has a booster on it. this one you have should be for a drum /drum set up because if you had disks you would need a proportioning valve. It also says the lines closest to the booster is for the rear and farthest away from the booster is for the front and the lines do cross each other, because it need to be plumbed this way as the master cylinder ports are staggered to get the front braked to hit first creating the dive you need.

good luck

Wow 3B, that's going above and beyond! Thanks so much for the help, I didn't even think they'd know!

My master cylinder is definitely at a level above all four drum brakes so at least I don't have that to worry about. But from the sounds of it I'll still need to install two 10 p.s.i. residual valves. Other than that, I already bought the plugs and all the hardware. I'll see what I can cook up. Thanks everyone for the help!