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New 1970 master cylinder leaking

Discussion in 'Early CJ5 and CJ6 Tech' started by 47v6, Nov 12, 2014.

  1. Nov 12, 2014
    47v6

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    So, I'm trying to figure out why my brakes are spongy. Did the full GM disk brake conversion all the way around and 2 PSI check valves. Everything is new. Bled my brakes at least a gallons worth. Put vise grips on the flex line to the front set and voila, nice hard pedal. So today I bled the front ones some more. look under the body and the "new" wagner master cylinder is leaking out the back. Guess I found my problem... maybe. Still stops well, but you need to pump the brakes once, then its fine. I hate new things that have quirks.
     
  2. Nov 17, 2014
    AKCJ

    AKCJ Active Member

    Fairbanks, Alaska
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    I have a 70 and I have disc brakes on the front.

    When I first put on the dana 30 with discs I had spongy brakes and like you went through a gallon of brake fluid. I even did a caliper rebuild. Finally a friend spied the problem - I had a drivers side brake caliper on the pass side (thanks to a PO). The bleeder was not at the high point so the air couldn't get out. I ended up taking the caliper off and holding it so the bleeder was at the top. After getting the air out I put it back on and it's still working years later. I do plan to replace it some day.

    During the above trouble I replaced the dual master from NAPA. It was expensive but an exact Lockheed replacement.

    I also sometimes have to pump once to get hard braking force. I assume that is because the master doesn't move enough fluid for calipers. Not sure of the best fix for this.
     
  3. Nov 20, 2014
    47v6

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    I hate the fact that I need to pump once to get a good hard brake pedal. really hate it. I have all new GM calipers, new flex lines and a new master cylinder. The disks are new, the pads are new. I have bled the brakes multiple times. I clamped off the flex tube to the front axle. instant hard brake pedal. removed the clamp, clamped one flex tube to one caliper, no hard brake pedal, the other, the same. clamp off both and its a hard brake pedal. Calipers are correctly oriented. The bleeder is directly at the top.

    I clamped off as close to the metal fitting end closest to the caliper to see if i had ballooning flex lines, no dice. Its in the caliper. Now, I have the same exact calipers oriented the same way in the rear. No problem there. Whats MY problem?
     
  4. Nov 20, 2014
    AKCJ

    AKCJ Active Member

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    Did you pipe the rear master outlet to the front brakes? I can't confirm it but I've read that volume output is or can be larger from the rear circuit on the master. It sounds like you have the same size (volume requirement) calipers front and rear. If you get a hard pedal when one end is clamped off that adds some to the above theory that one end of the master puts out more volume.

    On mine I have to do the double pump only if I haven't used the brakes in a short time before and I'm making a hard stop. Most of the time I don't need the double pump.

    What's the solution? The only thing I can think of is changing the master to one that puts out more volume. As far as I can figure having to double pump the pedal indicates that the master isn't putting out enough volume to fill the caliper. In my case I'm using the original design master that was made for drum brakes. I'm not certain what master you have.

    Just thinking. I guess a guy could try using calipers that take less volume instead of using a master that has more? This is theory only - I don't have part numbers. You would give up braking force but then you don't need as much force on the discs on the rears.
     
  5. Nov 20, 2014
    47v6

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    What I have is a 1970 cj5 drum brake master cylinder. I also have the 2lb check valves front and rear. I took out the caliper pins/bolts and tapped the calipers with a hammer, shook em around, tilted them back more but over the disk, waved a rubber chicken over them and bled them again. There is just no air in the lines or the calipers. They do work a little better and I can lock them up, but it takes a stutter pump the get the real high hard brake pedal. without the initial pump, they do stop the vehicle fine, I just have a longer pedal travel.

    I hate that. Everything is new. I can deal with a slight stumble off idle, a funny noise, even some slight driveshaft vibration. I really hate crappy brakes. maybe I'm brake OCD or something.
     
    Last edited: Nov 21, 2014
  6. Nov 21, 2014
    sterlclan

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    A disc caliper uses more volume than a wheel cyl. Master probably not producing enough volume might need a disc disc master.
     
  7. Nov 21, 2014
    mikieboy

    mikieboy Member

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    I had a problem like yours, all new parts. Did everything you have done. Finally replaced 2lb valve, brakes were better, but not like new brakes should be.
     
  8. Nov 21, 2014
    47v6

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    Well, that may be my problem. Anybody wanna chime in?
     
  9. Nov 21, 2014
    nickmil

    nickmil In mothballs.

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    If you installed a 2 lb. residual pressure valve sounds like one of a couple things. Defective valve, excessive runout on a rotor(s) pushing the caliper piston(s) back in too far, or maybe air trapped in the high point of a line or caliper somewhere.
    Fwiw, I ran the stock single reservoir master cylinder for 9" brakes for years with 4 wheel discs with GM 1/2 ton truck calipers with no issues. Pedal was a little lower than I liked but never an issue and could lock up the brakes at will.
     
  10. Nov 21, 2014
    47v6

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    As a reference, these brakes work worlds better than the stock 9 inch widow makers. My issue is that I have 1/2 ton GM brakes on a 2000 lb vehicle. i should basically be able to stop instantly. What my problem can be attributed is to air in the lines,but I have no air in the lines. The 2lb residual valves are new, front and rear and my spongy pedal is the same moving or not. Maybe I need to go hanging pedals and power brakes.
     
  11. Nov 21, 2014
    nickmil

    nickmil In mothballs.

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    My pedal was hard with no sponginess, just a little lower than I liked.
     
  12. Nov 21, 2014
    4xfun

    4xfun New Member

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    You did not mention if the rubber hoses to the brakes were new. If they are old, they may be swelling when you apply pressure.
     
  13. Nov 22, 2014
    47v6

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    No, they are new. I also clamped off the rubber lines closest to the caliper then closest to the steel fitting on the master side. The are not at all ballooning. The brakes are high and hard when doing this either way. The problem seems to be in the calipers. they are on with the bleeder pointing up, etc. I did everything everyone else did with the exception of power bleeding. It could be that the 2lb residual valve in the front is bad(new). Rear calipers are the same as front and not faulty in any way.
     
  14. Nov 22, 2014
    Walt Couch

    Walt Couch sidehill Cordele, Ga. 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    Swap residual valves and see.
     
  15. Nov 29, 2014
    47v6

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    Had to stop going down a hill in traffic when a car stopped quick in front of me. the oh cr@ap moment had arrived. The pedal sinks more than I like, but it stopped without effort, perfectly straight and no issues whatsoever. I guess what it comes down to is the amount of brake fluid that a caliper uses vs the amount a wheel cylinder uses is greater.
     
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