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1939 j4 cub brake master cylinders


Registered User
Talkeetna, Alaska
I have the old style 1 1/4" diameter brake master cylinders. A couple of the rubber components are worn out and i'm having a hard time finding a rebuild kit. I found a 1939 dodge truck rebuild kit on-line, but i have not found out what size diameter it is yet. Also, is 5606 going to affect the rubber & make it swell-up, or is regular brake fluid correct for this type of system. Thanks guys.
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58 pontiac or chev as I recall , car fluid. I might have the box the kit came in
All the J4 around here use automotive brake fluid. I use Dot 5 because the cheaper Dot 3 takes the paint off if it gets on anything. Check and see what's in your system now, if you mix the brake fluid with the 5606 it turns into goo.

I too use Dot 5, but good thing I have no leaks- it went from five bucks a quart four years ago to thirty!
Dot-5, ok. Thanks Cubx3, that will help alot if you still have the brand or part # of the rebuild kit. Thanks.
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Be aware that DOT 3, 4 & 5.1 fluids will absorb moisture, DOT 5 does not. Sounds like a good thing until you realize any moisture in your brake system will settle in the low point and corrode your equipment or form an ice slug in your brake system. If you have a vented system moisture is a given. DOT 5 is not generally recommended for street vehicles. http://www.raceshopper.com/brake_fluid.shtml Jim
Yep, i have heard of the one inch ones too, but mine are 1 1/4". I took both the pistons out of them and one has a different seal-ring groove height, so i think it will be best if i get a couple new kits, maybe a 3rd for spare. Theres an old military parts place in Anchorage called Pattsons, i stopped in and he had a piston that was correct, it was a Wagner part# & he's checking on a rebuild kit for me too. I have another week until i can get back to civilization, but that gives me time to research, thanks for all the help guys!
I used a 1958 chevy truck repair kit, the pistons were the wrong height and the bore depth was too shallow for the push-rod, the 1 1/4" diameter was correct so i was still able to make it work. (Raybestos p/n MK-33)