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Thread: Disk Brake Conversion for a 1954 CJ3B

  1. #11
    Super Moderator bmorgil's Avatar
    Join Date
    Dec 2018
    Northwestern Ohio
    Your thinking is correct AJ. The proportioning valve is the key to balancing front to rear brake force. Locking up the rears before the fronts will cause spinning, especially in a short wheel base. Front discs and rear drums were quite common in the 70's and 80's. The front brakes do much more of the work than most realize. The harder a vehicle is able to stop with the front tire brake combination, the "G's" it can pull from the front, the more the weight will shift to the front. Eventually if you can get the traction and the brake clamp, lifting most of the weight off of the rear tires altogether. I have seen a few tire brake combinations pull some serious G forces on a skid pad. In some cases you could adjust the rears almost completely out.

    To compensate for the pressure differential requirements of Drum / Disc, a combination valve is used to compensate internally. It does a few things to make the drums work smoothly with the Discs. I would recommend using the dual master cylinder recommended by the people you are buying the kit from. They had better know if their kit needs a proportioning / combination valve or not. In any event you described how to adjust a proportioning valve perfectly. You go out and lock them up. Fronts must lock first. For street cars we always tried to lock the fronts and barley drag the rears. When you are track adjusting a lot of factors come into play. That is the benefit of adjustable over non adjustable proportioning valves. Its not a big deal. I can tell you know what your doing. Don't let it scare you off if you want some nice brakes. Order up your favorite kit, put in a Dual Master cylinder and a combination valve and you will love it I am sure. You don't need anything fancy or adjustable unless you want that. A simple off the shelf 1970's era gm combination valve will do the trick.
    Last edited by bmorgil; 02-19-2021 at 09:20 AM.

  2. #12
    Junior Member AJ-MJ's Avatar
    Join Date
    Jan 2021
    Hopewell, Virginia
    Thanks again for keeping me on the straight and narrow bmorgil: I think I have forgot more than I know about fixing cars. But I do want good brakes! In the early eighties I was in the army and stationed in Germany. I bought a decent Mercedes 230 SL roadster. What I later discovered was that it did not pass the German inspection because of rust. Therefore, I hired out all the repairs and brought it back to the US. So that is when I discovered the need for good brakes on a CJ5 with over sized tires. While towing my Mercedes behind my CJ5 I had to do an emergency stop. I ended up in the meridian of a 4 lane highway and rolled/totaled both vehicles. I was lucky to walk away with only a scratch on my leg. The state troop said I was lucky that not many people walk away from roll overs. Ever since then brakes and brake maintenance has a pretty high priority to me. I will mostly likely call KW when I get to the point of rebuilding the brake system. I am pretty sure they should be able to provide some guidance since they sell the kits. I have one more related question. Is the DOT 3 brake fluid satisfactory or should I use the DOT 5? I understand the benefits of the DOT 5, I am just wonder the use the the old jeeps.

  3. #13
    Super Moderator LarrBeard's Avatar
    Join Date
    Nov 2014
    Ft. Wayne, IN

    DOT 3 vs, DOT 5 and 5.1 rake Fluid

    Quote Originally Posted by AJ-MJ View Post
    Is the DOT 3 brake fluid satisfactory or should I use the DOT 5? I understand the benefits of the DOT 5, I am just wonder the use the the old jeeps.
    The main advantage of DOT 5 fluid is its higher boiling point - something you might have concerns about in your German Mercedes or other really high performance car, but probably not in a CJ.

    Here is a cut and aste from a pretty good explanation of the differences:

    DOT 3 is always the cheapest optionóDOT 4 is about 50% more expensive than DOT 3 and DOT 5 is about two times more expensive than DOT 4. Some owners mistakenly assume that the higher cost of DOT 5 equates to better performance, but this is not always the case. In fact, sometimes the exact opposite is true. To get a better handle this, we need to take a crash course in chemistry.

    Common brake fluids such as DOT 3, 4, and 5.1 are mixtures of glycols and glycol ethers while DOT 5 is silicone based. DOT 3, 4, and 5.1 are hydroscopic, which is a fancy way of saying they absorb water. We know what you are thinking--brake line moisture is evil! It is true that we try to keep our brake systems "dry", but over time, even a buttoned up brake system with tight seals and new lines absorbs moisture. The key here is what happens to that moisture after it enters the system.

    DOT 5 doesnít absorb moisture. But even though DOT 5 doesnít absorb water, it canít/wonít prevent moisture from entering the brake system. And since the water isnít absorbed by DOT 5, moisture puddles and causes localized corrosion within the brake system. As funny as it sounds, DOT 3, 4, and 5.1 absorb moisture, which in turn eliminates the puddling that can cause corrosion. More importantly, when brake fluid heats up, water trapped inside the brake lines (but not absorbed by the brake fluid) is converted from liquid to vapor. Steam compresses easier than liquid. With this in mind, imagine barreling down the road at high speed and hitting the brakes. When the hydraulics sends DOT 5 fluid through a pocket of steam in the line, that drop in pressure creates a soft pedal . (Probably not a Jeep concern..).

    Most folks know they aren't supposed to top off DOT 3 or 4 brake fluids with DOT 5, but don't know why. The answer goes back to the chemistry. Combining even trace amounts of a glycol-based brake fluid with DOT 5 can cause the two incompatible fluids to gel, resulting in poor braking. Converting to DOT 5 also requires thorough flushing and removing ALL traces of the old fluid to avoid seal damage.

    This is another case with an Old Jeep where newer is not necessarily better.

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