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TOPIC: J4 brake fluid

J4 brake fluid 11 months 1 week ago #842

  • e2wallace
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I have to rebuild the master cylinders in my 1939 J4. The cylinders are Wagner, different than the J3, and similar to cylinders used in Dodge trucks of the same vintage. I saw an article from Univair that said parts are available from NAPA for rebuild, but seals are compatible with DOT4 fluid which is not compatible with the expander tubes. Anyone have experience with a brake fluid that is compatible with these cylinders and the expander tubes? I'm not sure what the original fluid was.
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J4 brake fluid 11 months 1 week ago #843

There is an article in Cub Clues #108 on the subject of brake fluids. According to that piece, the pre-war J-4's and J-5's used Lockheed 21, an automotive brake fluid. In 1942 Piper went to Univis #40 which appears to have been the same as Mil 5606 which we still use. If you can determine by color or other means what type fluid your system had in it, that may help. The article recommends first cleaning the brake bladders and the rest of the system with denatured alcohol, and then use silicone brake fluid. Apparently the silicone fluid is compatible with both natural and synthetic rubber parts. The article also cautions that mixing some types of the old and new fluids can result in a gooey mess in the brake system. Hence the alcohol clean out first. You could also test one of the new seals in a jar of the fluid you plan to use. A couple of days soaking should tell the story if they are not compatible.
Let us know what you find out. I may wind up rebuilding a set of master cylinders for a J-5 myself.
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