Disc-Drum M/C for 4 wheel drums & other brake concerns - JeepForum.com
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Unread 07-16-2013, 08:58 AM   #1
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Join Date: Jul 2013
Location: quezon city, philippines
Posts: 2
Disc-Drum M/C for 4 wheel drums & other brake concerns

Hello All!
I have an early 1950`s tall hood jeep (a japanese version of the CJ-3B, called a Mitsubishi-Willys) with a relatively modern 2.7 liter Mitsubishi 4DR5 engine transplanted in it. Anyway, i`m on a budget and for now would like to try to use the M/C i have on hand. My problem concerns my braking system.

GIVEN: I have all four drums using 1" bore wheel cylinders. I also have one of those japanese made Dual circuit aluminum master cylinders (15/16 " bore) designed for rear drums and front disc brakes. My pedal system was also modified such that my M/C is now mounted high on the firewall. All wheel cylinders and the master cylinder are only about a year old. I have no leaks anywhere and bled the system well. The original stock brakes had 1" bore Front wheel cylinders and 3/4 " bore rear wheel cylinders and a 1 " bore M/C.

PROBLEM: it takes me several pedal strokes to get a high firm pedal everytime i hit the brakes. As long as i keep my foot pressure on the brake pedal, it stays high and firm, then when i release, another three strokes of the pedal to get the pedal high and brakes strong, and the cycle repeats.

I understand that this is likely due to having a too small bore M/C and what i assume is having no residual pressure valve going to the front brakes (since my m/c is for a rear drum / front disc system).

SOLUTION ? ? : 1- Attach an inline residual pressure valve for the front brakes (10 lbs right?) and put in the correct 3/4 " bore rear wheel cylinders. I`m hoping the brakes will be good and firm on my first pedal stroke, but predictably lower since the present M/C bore is undersized by 1/16 ".

I`m also hoping that using the original 3/4 " rear wheel cylinders will improve the braking dynamics. Pls note my transplanted 2.7 liter 4dr5 diesel engine, i wonder if its heavier than the original engine was, and if so might have any kind of profoundly negative effect on the braking dynamics? What do you guys think?

I see a lot of stuff on the net adapting a dual M/C designed for 4 wheel drums and being used for front disc/ rear drum applications, but never seen adapting an M/C designed for a front disc/ rear drum to be used for 4 wheel drums, hence my post. Also seen formulas / ideas for calculating brake system component sizes for disc-drum and disc-disc systems (Pirate 4x4 Brake bible, etc..). Or maybe i`m not looking hard enough? Anyway, your comments would be welcome, as well as a website for my concerns. I`d rather not do all the math and find out it was designed with a disc-drum or disc-disc setup in mind. The "Brake bible" type of info is ok for me if it deals with a drum-drum setup, otherwise comments pls. Thank You!

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