S120 single to dual circuit drum conversion

[284International]

I have a 1956 s120 4x4. It has a Dana 44 front and the IH rear axle, with the big 6 lug bolt pattern.

I just got the vehicle running well, and now am ready to dive into the brakes before taking it for a test run. The previous owner said it would need a new master cylinder; so far everything he's told me was accurate so far as I determine. My plan is to replace all the hard and soft lines, wheel cylinders and shoes on this 4 wheel drum system, along with the master cylinder.

I want to go to a true dual circuit master cylinder, just for the safety factor, since I intend on making this frequent and regular driver on fairly busy roads. I'm not especially worried about the drum brakes per se, but since I'm going to be replacing things anyway, I'd prefer to go to a dual circuit system.

The master cylinder on these is kind of frame-mounted. Are there good choices for a fairly simple to adapt dual circuit drum brake master cylinder?

I have a hydrovac setup that came with the truck but not attached, if that is helpful/beneficial in the conversion.

If need be, I can fabricate and weld a bracket for another style of master cylinder, if it will merely clear everything and fit into the spot. The preferred option, obviously, would be to find something that's more or less bolt-in.

Thanks for the wonderful forum, I'm glad to be here.

 

[Bruce A. Frank]

I am going to have to ask a couple of questions so I am sure we are talking apples and apples. When you use the term dual circuit, are you referring to a master cylinder with independent feeds to the front brakes and the rear brakes, the common system used on later model cars?

I drove my first s120 with the original type master cylinder with one pressure bore feeding both front and rear brakes from a single output to a "t" fitting. I put 250,000 miles on that vehicle and never even has to rebuild the mc.

But, with my "new" s120 I got to thinking about the safety advantages of the newer "split" systems that feed the front and rear separately. I bought a new mc that incorporates the front/rear design. And a vacuum pressure booster. But, I've not installed it yet be because I am trying to decide whether to adapt this new mc/booster to the floor peddle, or convert to a hanging peddle.

If you get back to me, I'd also like to discuss the slave cylinders and some differences I have found between my first s120 and the one I have now.

 

[284International]

Bruce,
thanks for the reply, I'm sorry it took so long to get back to you.

By dual circuit brakes, I mean the modern style master cylinder that independently feeds the front brakes and the rear brakes.

I ended up ordering a master cylinder that is as you describe and matches what I assume to be the original: a single pressure bore master cylinder whose output feeds a junction that directs pressure to the front and rear wheel cylinders.

I am very interested to hear about the differences in wheel cylinders and the changes you noticed in your vehicles. The wheel cylinders I received were similar but not identical to the originals, and didn't quite match up as nicely. The bleeder screws on the front axle in particular were obnoxious to access.

What master cylinder and vacuum boost do you have?

 

[Bruce A. Frank]

I put my set up together off ebay. All new parts. I'll try to post a picture later today.
======================

sorry for the delay. I bought the dual diaphragm booster with a dual/split master cylinder attached for about $50 on ebay. My original intent was to install front discs (yes I did find the parts to convert a closed knuckle axle) so I bought a unit with a disc/drum master cylinder. I later realized that it was going to be a while before I'd have the money to buy the parts for that disc conversion. I needed better brakes now.

Back on ebay I found a commercially overhauled split master cylinder designed for drum/drum for only $25. It required a little effort to pull the original off the booster and install the new drum/drum mc. Which included having to fabricate a new longer pushrod between the booster and the mc.

This photo shows the numbers of the bendix master cylinder. If you look it up that mc is used on several four wheel drive all drum vehicles. That number will cross reference to other available brands. Btw, the two bolt mount mc is, I believe, called a GM mount.

You'll have to fabricate the mounting brackets or find them on the web. Many companies have the hardware to fit boosters to any vehicle.

An example of the style of bracket from ebay:

Or this:

(above image May not last if the item is removed from auction )

another choice:

 

[Bruce A. Frank]

bruce,
thanks for the reply, I'm sorry it took so long to get back to you.

By dual circuit brakes, I mean the modern style master cylinder that independently feeds the front brakes and the rear brakes.

I ended up ordering a master cylinder that is as you describe and matches what I assume to be the original: a single pressure bore master cylinder whose output feeds a junction that directs pressure to the front and rear wheel cylinders.

I am not sure I understand the part about feeds a "junction." there should be to separate ports on a dual master cylinder, like the one in the picture in my previous post above. The port closest to the booster feeds the front brakes and the other port at the far end of the mc feeds the rear brakes. There are two separate cylinders inline in the mc body sealed on the same shaft. If the seals fail on either section, front or rear, the other still provides braking power.