Apparent Leak in My New Master Cylinder--Thoughts - NCRS Discussion Boards

Apparent Leak in My New Master Cylinder--Thoughts

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  • Paul S.
    Expired
    • April 6, 2010
    • 148

    Apparent Leak in My New Master Cylinder--Thoughts

    This Spring, I converted my 67 from standard to power disk brakes with all new calipers, brake lines, DC master cylinder and brake booster (obviously) from CSSB, Inc. and filled with DOT 5. I finished the initial install at the end of April, and had a couple of leaks pop up afterward which I resolved. Then I began seeing about a fifty cent piece sized drop of brake fluid that lands on the floor between the master cylinder and engine block, well back from the wheel to roughly the middle of the master cylinder.

    After a lot of trial and error, I'm convincing myself that it is the master cylinder leaking--I have pumped the brake pedal (car off) for 10 minutes which equates to 1,000 cycles (yes I counted), and couldn't replicate the drip. But go out and drive the car for about an hour, return to the garage, and shortly thereafter, the dreaded brake fluid spot once again. After each test drive, I would work on the master cylinder lines while still hot, getting them as tight as humanly possible, and checking for fluid along the front line that goes to the brass block at the left front tire, looking for a leak there.

    The flare nuts on master cylinder lines feel a bit slick, and I cannot rule out some residual weeping but nothing that would account for a big drop of fluid and it's location on the floor--in fact I put a paper plate below the master cylinder lines during my numerous 1000 pedal pump tests, and after last night's drive. 10 minutes after shutting the car down, I see the brake fluid on the floor yet the paper plate is clean.

    I was surprised to find last night that brake fluid was pooliing on the bracket behind the master cylinder--e.g., the bracket which holds the brake pressure switch/block combo was wet. The fluid was on the top and bottom edges of the bracket between the two master cylinder mounting nuts--enough wetness that when I swiped my index finger on the bracket I came away with a very clear smear of brake fluid. Also found some wetness on the engine side of the master cylinder in the area between the two wires that hold the cap down. I cleaned everything up and it is still dry this morning everywhere. So the leak is happening when driving and braking, that much I have concluded.

    I removed the master cylinder mounting nuts so that I could pull the master cylinder away from the booster, and it's dry back there. So I am fairly sure it isn't leaking out through the master cylinder piston.

    Can the master cylinder cap allow leak down of brake fluid like what I'm seeing? Does the brake fluid slosh that much with enough force to come up over the top between the cap gasket and the master cylinder casting lip?

    I've siphoned down the fluid level in both the front and rear reservoirs so that at the highest point the fluid is more than 1/2" below the top of the master cylinder. And the front reservoir on the up hill edge (closest to the radiator) is more than an inch below the top edge.

    I've also heard of porous master cylinder castings that leak. How does one tell if the master cylinder casting is bad vs. the leak being the cap?

    Finally, does anyone think that air flow on the road could carry the brake fluid up and under the rear of the master cylinder at the bracket where I'm finding it? That seems impossible because I don't see a spatter pattern on the fender or under the hood that you would think would happen if that much brake fluid was leaking from the brake lines and landing on the bracket between the two master cylinder mounting nuts.

    Just trying to figure out what to tell CSSB is defective and to make sure I'm not missing a leak that is traveling on me.
  • William C.
    NCRS Past President
    • May 31, 1975
    • 6037

    #2
    Re: Apparent Leak in My New Master Cylinder--Thoughts

    Start by asking them if they have any special suggestions for Dot 5. I believe it is a lot more difficult to seal up the system with dot 5 than dot 7.
    Bill Clupper #618

    Comment

    • Gene M.
      Extremely Frequent Poster
      • April 1, 1985
      • 4232

      #3
      Re: Apparent Leak in My New Master Cylinder--Thoughts

      Originally posted by William Clupper (618)
      Start by asking them if they have any special suggestions for Dot 5. I believe it is a lot more difficult to seal up the system with dot 5 than dot 7.
      What is dot 7? ...........something new I don't know about?

      Comment

      • Jim T.
        Expired
        • March 1, 1993
        • 5351

        #4
        Re: Apparent Leak in My New Master Cylinder--Thoughts

        I have been using DOT5 in my 1968 and 1970 Corvettes for 25 years or more now with no sealing problems. I did put new seals in the front calipers of the 1968 after using silicone DOT5 for about 23 years. One thing I like about DOT5 is that you can see the bottom of the master cylinder through the fluid after all these years. I will continue to use it.

        Comment

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