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Upgrading a Single Master Cylinder into a Dual set up

• CATEGORIES: Builds, Features This site contains affiliate links for which I may be compensated.

A reader ask me about replacing the stock single brake master cylinder, found on many early jeeps,  for a dual master cylinder.  Can it be done?  It turns out there a variety of threads and pages on this alteration.  Here are some of the links I found if you are interested in this mod.

4) Herm’s conversion kit:


(Image from the CJ-2A Page)


9 Comments on “Upgrading a Single Master Cylinder into a Dual set up

  1. Pingback: Upgrading / Changing brakes on older jeeps | eWillys

  2. mark massey

    Steve, good one. I wish I had this before I repaced the system a few months back. The orginal works fine but the backup safety of the dual system is key.

    here’s an idea. I’ve got orginal 48 axils with 5:38 and want better economy etc… what’s the concensus on the question:

    Overdrive vs diff. gear swap vs complete axil swap?

    I’m just too new to the motorhead world to break it down on cost/benefits.

    Just a thought, thanks. Mark

  3. deilers


    That’s a good question which doesn’t have a very simple answer.

    One nice feature with the overdrive is that you can buy one, easily install it, and test it out. If it works for you, than it’s a easy solution. If it almost works, but the gearing still isn’t right, you can always use it with any gearing. Finally, if it doesn’t work, its resale value is pretty good. It’s important to note that my research on warn/saturn overdrives indicate that to make them last, you don’t want to use it all the time; but, I have no personal experience using them.

    When it comes to the gear swap vs. the axle swap, and you are unconcerned with strength issues (swapping the 70s CJ axles provide higher spline counts and stronger axles), then these are questions you may want to consider:

    1) Do I want to keep the stock axles (some people want to retain the originality)?
    2) Can I find the gearing options I want at a reasonable cost for my housings?
    3) Can I locate a set of axles in good condition with the gearing I want?
    4) While I’m making these changes, do I want to upgrade my brakes (sometimes entire axle units come with brakes, saving money)?
    5) What gearing do I want?

    Depending on the engine you have, and I can’t remember what it is, 4:27 gears with an overdrive or 4:11 gears and lower without an overdrive may work the best. Old, beat up Wagons, trucks, IH scouts and Broncos can be good sources for these gearing sets.

    When I was making this decision at the start of my build, I found a great chart that outline various Dana 44 setups in different vehicles and the kinds of gearing that was offered, the width of the axle housings, and more.

    So, to answer your question about the cost/benefit decision, you need to be more specific about the gearing and housings you want and then see if you can find an existing axles as a good price that will meet these specifications. If you can’t find these, you may be better off swapping the gearing (though it’s not a simple task).

    Maybe that helps?

    – Dave

  4. mark massey

    are you kidding! thats the best quick and dirty I’ve ever read. Thanks so much Steve.

    As long as I’m amongst greatness may I ask one other?

    1956 GM 256 v8, 2bbl. D25 front, D41 back. T98 w/ T18. Decent v8 conversion (long ago and by unkn persons) with some fabrication on both drive shafts.

    One full wheel turn = 2.2 approx. turns on the axil yoke = What’s the gear ratio?

    I think my girl had some work done on the original 49 gear set. I’m cruising at 55 with a little left so I can’t believe it’s 5:38. Any wise thoughts would be appreciated. It’s a new mystery every month, that’s what makes it fun. Have a safe weekend.

  5. deilers

    Well, it sounds like you might have 4:27 gears. With the rear end jacked up, rotate the wheels twice (2 times). Assuming you don’t have a locker, then the amount the yoke turns is your gear ratio.

    Since you had about 2.2, which probably means it was maybe 2.15), just double that and you get 4.3 or 4.27 gearing is my guess.

    I think you’ve got a nice combo there. 4:27 gears + the T98 gives you a nice trail ratio and not a bad road ratio. So, I’d toss a warn/saturn OD on the back of the TC for those occasional highway trips.

    – Dave

  6. mark massey

    Ureaka! Best news in a while. This is a late 60s rebuild so I’m begining to think the guy knew what he was doing. It drives surprisingly well as far as this novice can tell.

    Can you trust Ebay for a used OD or would you scour the salvage yards and axil shops of East Houston? I don’t see them come up that often. Of course there’s always a new setup but that’s a chunk of cash for the cheepster Jeepster.

    I really appreciate the info Steve. I hope you get the “Bisquit” out of town this weekend. I look forward to those posts.

  7. deilers

    I’d be pretty surprised if you can find an overdrive in your local salvage yard, but you never know. I’ve seen them appear more often on craigslist than on eBay.

    Herm sells new overdrives for $875 ( Used ones appear for $400 – $600 on craigslist. I’ve seen a variety of old jeeps off of craigslist that include the overdrive and linkages. You can also search for old IH Scouts. You’d need to buy an old jeep or scout and then resell the jeep or part it out. However, there’s no guarantee that a used OD is any good — you may end up having to get it rebuilt.

    You’ll also need to make sure you match the pto gear with your transfer case. Check out the link above to herm’s website to see all the different options. For example, here’s a PTO for $400 in OKC, but it appears to be a 29 tooth 10 spline ( which likely won’t work for your application.

    When getting the OD, make sure you are getting a Warn/Saturn or equivalent, as some Overdrives (Borg-Warner Rancho for example) actually install along the rear output shaft rather than in the PTO location of the transfercase.

    – Dave

  8. mark massey

    ok, last stupid question of the month.

    How do you check to see for sure what tooth/spline I have? I think I could access the PTO cover plate and shine a light in there.

    I’m not in a hurry so I’ll ferret out a good deal in time.

    Thanks, Mark

  9. deilers

    Here’s a discussion on the topic form the CJ-3B page.

    Count the teeth on the transfercase input gear (the gear you see when you take off the PTO plate.

    NOTE: Read Herm’s advice on purchasing a used overdrive. He knows far more about this topic than I.

    – Dave

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