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Looking for info on this swap... I have a air leak in my 83 CJ power brake booster, i need a new or reman booster of course but im thinking take the opportunity to upgrade (if it is one).

Anybody done this YJ to CJ booster swap or have good info please share. thanks 

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Well i did the swap, the pedal requires less effort and braking is improved, i retained my master cylinder which required slotting the mounting holes and cutting the casting at the bottom. The drive rod behind the master needs to be swapped or modded. Using the taller YJ master would have hit the rad support rod and put the lines on the carb side. Link rod inside simply hooks up and the brake pedal and is maybe a we bit lower.


















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GoatBoy4570...Interesting and great photos!  Not exactly a "bolt-on" swap, but you made it work.

These would be some concerns:

1)  Aside from making sure the master cylinder fits the booster properly, it must seal out moisture.

2)  The booster pushrod and master cylinder piston must align without any bind.  You mention this.

3)  Brake pedal travel should be able to move the piston over its range without the brake pedal hitting the floor first.  The pedal should also travel to the floor.  (Test without any fluid pressure in the system.)

4)  Adjust the booster pushrod so that the master cylinder piston can retract completely with the pedal released.  Use the CJ rod-to-piston adjustment specs.  This adjustment assures complete or proper retraction of the master cylinder piston when the pedal is released. 

5)  Brake pedal height on a CJ is adjustable.  You can raise or lower the pedal height to provide full range of travel and full retraction of the piston.

Note:  The piston must retract properly to prevent blocking the compensating ports in the master cylinder.  If the piston and seals do not clear the compensating ports when the pedal is retracted, fluid can become trapped in the brake system and cause brake drag or lockup.

Is the YJ booster a significant advantage?  Others have upgraded to aftermarket dual diaphragm boosters with high volume G.M. style master cylinders.  60Bubba, a fellow member, did such an upgrade on his CJ-7:


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Thanks Moses for the added info on my swap, the devil is in the details as they say.

60Bubba uses the 8" DD booster / MC kit that is readily available on Ebay. From my research that engineered kit is the final word (the bomb) next to a hydro setup.

It should be mentioned as well that OEM grade (organic) brake pads and shoes are marginal at best and metallic or carbon metallic are mandatory. All things considered the booster swap and premium pads (once they are burned in) made my CJ a much safer ride. This is an improvement but for those wanting more or running bigger than 33" tires the DD set up is the way to go.

The photos i uploaded are out of order, the push pin from the YJ booster is shown first in its modified (shortened) form. The push pin is threaded inside (1/4-24) halfway down, when i cut it shorter with a hacksaw i ran a bottoming tap down to make some more thread, i then screwed in the hardened ball end.

The push pin is semi floating semi self aligning in the front of the booster, the push pin locates in a cup with a rubber floor inside the booster and can accommodate a small amount of misalignment, lets say manufacturing tolerance. My plan is a new MC with the mounting holes more precisely slotted and a cleaner cut on the bottom casting.

The brake pedal push rod could be a bit longer and some folks may like the brake pedal a bit further back but i like it were it is.

Looking at the pictures the YJ master would have just cleared the rad support rod so the only real issue would have been the lines on the carb side.

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Goat Bot 4570...You've put a lot of thought into this booster swap.  Would you say that the YJ booster's volume provides a net gain over a stock CJ booster?  Would the OEM CJ booster, if rebuilt properly, provide similar performance to the YJ booster?  We can see in the side-by-side view of the two boosters that the YJ booster is distinctly larger in diameter.

As a rule of thumb, a booster is never a substitute for poor braking performance at the master cylinder or wheel brakes.  A classic case is the vintage WWII Jeep MB through mid-'sixties CJ models with four-cylinder engines.  They have inadequate 9-inch diameter wheel brakes.  In the day, some would install a vacuum booster unit inline to increase the pressure at the wheel cylinders.  (A Bendix Hydrovac was popular on 1950s bread trucks, vans and medium duty trucks with hydraulic brakes and a through-the-floor brake pedal.) 

How this works:  The master cylinder sends a signal to the Hydrovac; the Hydrovac has an engine vacuum source and diaphragm; like any vacuum booster, Hydrovac boosts the hydraulic pressure to the wheel cylinders (lowering the brake pedal pressure/feel in the process).  For a given pedal pressure, the hydraulic system applies more fluid pressure at the wheel cylinders.

The real problem was the 9" x 1-3/4" brake shoe size and tiny brake drums!  Boosting the hydraulic pressure decreased brake pedal effort, but the result was like two feet standing on the pedal.  The hydraulic pressure went way up; however, braking was only marginally better.  Worse yet, the wheel cylinders generally blew out (especially if worn!)—and on a single outlet master cylinder braking system, such a prospect is highly hazardous!

A high output booster is never a substitute for poor wheel brake sizing, worn/scored shoes and drums, or rotors and other brake system weaknesses.  In your case, you recognized the inadequate brake pad material and compensated for the problem.  You didn't just boost the brake hydraulic apply pressure.


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10 hours ago, Moses Ludel said:

Would you say that the YJ booster's volume provides a net gain over a stock CJ booster?

It does, i would estimate around 15%. I can screech my front tires now on a hard stop were before i couldn't, pedal response is faster too.

I will admit my method of engineering was, lets try it and see if it works after reading somewhat conclusive information on the net.

I fully realized that the CJ and YJ have significant design differences in there booster setups. The brake pedal after all is nothing more than a second class lever and as it happens the CJ has two 2nd class levers with the other one between the brake pedal and the booster.

So we have a double underdrive situation multiplying mechanical effect to the back of the booster. My thought was the engineers at Jeep eliminated that on the YJ and must have made an improvement somewhere or they wouldn't have done it. 

All i can conclude is the booster must be a better (dare i say way better) design, its dimensions are slightly bigger with a lot more volume on the vacuum side and the valving is way more responsive, these changes although subtle seem to be very effective.

10 hours ago, Moses Ludel said:

Would the OEM CJ booster, if rebuilt properly, provide similar performance to the YJ booster?

I seriously doubt it, the Yj is an improvement but i think Jeep finally went to the double diaphragm in 95 because they wanted to (or had to) go from half decent to pretty good.

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Helpful information for members and guests, GoatBoy4570!  This is a cost-effective solution if done properly.  There are fitment and safety concerns with any brake modifications, but the gain looks obvious:  YJ parts are far more robust.  

The CJ power booster system left much to be desired though it certainly stood as an improvement over the non-power braking system.  The original modest boost might have been intentional considering the quirks of a shorter wheelbase vehicle like the Jeep CJ-5 at 84".

Worth sharing, I used new CJ disc rotors at all four wheels with S-10 G.M. calipers on the '55 CJ-5 depicted in my Jeep CJ Rebuilder's Manual: 1946-71.  A later dual master cylinder with ample piston displacement was also installed.  Without a booster and using the original through-the-floor brake pedal leverage, the vehicle stops very well.  At an 81" wheelbase, braking is as responsive as tolerable!  A manual proportioning valve to the rear brakes was part of my build along with an upgrade of brake tubing size.  The point is that the booster or a booster's capacity is only as effective as the sweep area, drum/rotor size, shoe/pad material as you state, and the condition of the wheel brake systems.

Thanks for the details and great photos, if you have impressions to add as your testing unfolds, others will benefit!


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  • 4 weeks later...

   Its been 3 weeks and i thought i would update this thread with some seat time feedback.  I have to say im a lot happier with my brakes and they seem to have burned in a bit more and gotten even better.

The one caveat if it is one is that under panic stop hard braking there is a bit of spring in the pedal, i suspect this is the firewall flexing a bit were the booster mounts, stands to reason really this has to be the strongest part of the firewall.

One thing i dont have to do anymore is hang back in traffic all the time, i was leaving extra stopping space because my brakes were so shitty were now i can do the 2 second rule at any speed, just like a normal car.

I have to say i have to highly recommend this swap, although not a strait up bolt in, with the right tools quite doable and worth doing


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Thanks for the continuing feedback on your brake upgrade/modification!  Other CJ owners will find this useful.

Is this "flexing" at the firewall or possibly a case of brake pedal pulsation?  Out-of-round rear brake drums or front rotors with too much runout can cause brake pedal pulsating or pedal pumping under hard braking effort.  Too much hub wobble (front wheel bearings loose or a loose rear axle nut) can also create pedal pulsation.

Something to consider...


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