Spongy pedal with Manual front disc brakes/rear drum brakes

Brakes, Steering & Suspension

  1. Justin Smith

    Justin Smith Well-Known Member

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    This may be a little long winded but bare with me. I have a 1970 super bee with manual disc brakes in the front and drum on the rear. The brakes haven't ever really worked right since i got the car. I have a really spongy pedal, however, if i pump it up about 4 times, its gets real firm and does not bleed off. if I let off of it for a few minutes, ill push the pedal again and it goes right to the floor, i pump it back up about 4 or so times, it builds firm again. When i purchased the car, it had a drum/drum MC on it (Wrong MC for the application that was on it), I replaced it with factory style manual disc/rear drum MC from Classic Industries. I bench bled the master cylinder before installation, and bled the brakes several times, several different ways, going through ever bit 3 quarts of DOT 3 synthetic high temp brake fluid. I have no air in the lines, and I have looked over all the fittings in the brake system and there are no leaks anywhere. pedal is still spongy until i actually pump them up. What i have noticed is my front brake calipers are still engaged to where i cant turn the front wheels. the rear drums are fine, I push the pedal, they stop, when I release the pedal, I can turn them. I did do an adjustment on the rear drums to rule out the out of adjustment of the drums. So what is keeping my front calipers still engaged? could the proportional valve be bad? Maybe calipers are both sticking? Have any of Y'all run into this same/similar situation with a Manual disc front/ Rear drum set up?

    Thanks a bunch!
    Justin
     
  2. hunt2elk

    hunt2elk FBBO Gold Member FBBO Gold Member

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    Sure sounds like you have air in the system. Are the bleeders on the front calipers on top? One guy on here had a master cylinder leaking internally and it was acting like yours is.
     
  3. Justin Smith

    Justin Smith Well-Known Member

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    Yes, the brake bleeders are on the top.
     
  4. Justin Smith

    Justin Smith Well-Known Member

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    It was an expensive Bendix Factory replacement MC from classic industries. I reckon it could be MC but dang, it was like $130
     
  5. dcala84141

    dcala84141 Well-Known Member

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    Yes he did
    He ended up buying a wilwood master 15/16
    and a 10lb. Residual pressure valve on the drum circuit and it solved his problem which sounds just like what your going threw
    Good luck
     
  6. Justin Smith

    Justin Smith Well-Known Member

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    I plan to call Classic Industries and talk with them about it, maybe I have a defective MC. Like i said I bench bled it and bled the system many times, and I have no air coming out, its gotten to a point it seems i am just wasting brake fluid.
     
  7. Justin Smith

    Justin Smith Well-Known Member

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    by chance you have part numbers? if i cant get this resolved, i may just have to break down and buy the wilwood set up
     
  8. R413

    R413 Well-Known Member

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    Bleed with this tool. Open the bleeder 1/2 turn, Put black end on bleeder clear end in a jug and pump the pedal slowly. It does not aerate the fluid. Works like a champ! $5 At Napa. Evercraft 776-9089.

    one bleeder at a time!
    B986CAD1-C5AC-4E8C-BB0C-8B33831B650F.jpeg
     
    Last edited: May 2, 2020
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    • Ranger16

      Ranger16 Well-Known Member

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      I have heard of the rubber brake lines deteriorating internally and acting like a check valve, which would cause that situation, but I don't think it would cause a spongy pedal.
       
    • Justin Smith

      Justin Smith Well-Known Member

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      I will try that, thanks! dumb question, should I get multiple of them or just get 1 and do 1 bleeder at a time?
       
    • dcala84141

      dcala84141 Well-Known Member

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      Search manual disc/drum break headache
      Goes by the name Gyratingyak
      Read the thread sounds like what he went through you are going through
       
    • joemish

      joemish Well-Known Member

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      Classic Industries is a parts supplier they are not mechanics.
       
    • coronetguy

      coronetguy Member

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      Two things. Still jave the brake lines running diagonally or are the discs on front on one circuit?
      Do you have a proportion valve?
      I switched my 66 Coronet from a single MC to a dual bit the front was on one circuit and back on another with a prope
      Proposening valve. Brakes pit you through the windshield.
       
    • jim94746

      jim94746 Member

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      • BB BELLA

        BB BELLA Well-Known Member

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        if the front rotors won't turn when brakes are released sound like you have a residual valve in the front circuit,when it should be in the rear where it's needed! you sure you don't have the lines switched at the master
        have the rear drums been machined true and the adjustment light drag!
         
      • Bb69gts

        Bb69gts Member

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      • Bb69gts

        Bb69gts Member

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        Just a thought.a lot of master cylinders that get rebuilt come back without the residual valve part! I have seen it as I use a bendix disc brake master for my 69 GTX factory disc brake car.
        Another idea to check is that I used 73 A body disc brakes (manual) to convert my 66 charger. You have to match the master to the application or brake pedal feel and performance is noticeable.
        Make sure you have the right MC for the right application.
        So far, I like the post about the residual valve for your front caliper problem
         
      • AJ/FormS

        AJ/FormS Well-Known Member

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        You have several problems and air in the lines is not one of them.

        Before you start, verify that your brake booster is functioning correctly. Engine off, pump the brakes about a half-dozen times to empty the vacuum chamber. Now, press down lightly on the pedal about an inch and a half and just hold it there. Start the engine. Immediately the pedal should start to drop, as the manifold vacuum evacuates the chamber. This would be normal. Going to the floor is not, but we'll get to that later.
        Next,slip your foot sideways off the pedal, and it should return to it's parking spot, probably making a nice thud. Reach down and pull up on the pedal, it should not move. If yours is behaving like this then it is fine and we can move on.
        Advisement
        Keep in mind that the calipers always drag to some degree. The only way that the pads retract is thru seal-retraction, which is the Square section seal inside the caliper doing it's job.But they should only drag very lightly.So if you can physically turn the wheel, with only a lil effort, the dragging may be normal, so you can skip Page-1
        Page-1
        #1 is the sticking calipers; you have to solve this first. This has nothing (probably) to do with your crappy pedal, but you have to fix this first.
        This problem can be either; hydraulic, mechanical, or operator error;
        First the hydraulic; the fluid is not returning to the M/C. It's just that simple. But the why of it is not so simple.
        1) As already said, this may be a residual valve.
        2) As already said, you may have plumbed it wrong; the frontmost reservoir gets plumbed to the rear brakes.
        next the mechanical;
        3) the pistons are stuck in the bores
        4) the hoses are internally collapsed
        5) the square section "o-rings" in the caliper are not functioning properly
        6) debris inside the line at a critical juncture
        then operator error;
        7) the M/C pushrod is too long and the piston is not releasing the fluid back into the reservoir.
        hunting;
        Of these; 2,3,4,and 5 are an easy test; 1 and 7 are a lil messy; and 6 is a turd so leave it for last, you probably will find the problem elsewhere.
        As to
        #2, just go look,lol
        #3,4,5,and 6; you can do all at once. Dismount a caliper, one atta time, and slowly push the piston back in with a big channel-loc pliers; not a clamp. If it moves,obviously it ain't seized, and the hose is good, and the fluid went back to the reservoir, pretty much ruling out debris..If it feels springy as you clamp and release, then the rubber rings are probably Ok. You may be able to see the piston moving in and out as you clamp and release, just a few thousands of an inch. You don't have to bury the piston in there, stop after you verify the O-rings are working. Re-install it and pump up the pedal. Then check the other side in the same way.
        #1, you'll have to go look
        #7, I don't think this is your problem so skip it for now.
        Ok by now, you should have discovered your problem.
        Notes;
        The combination valve has nothing to do with this, if you have plumbed the front brakes to the NON-PROPORTIONING, TOP end of it. The top side is nothing but a distribution block.

        Page-2
        I'll tell you what;
        After 4 pumps,you do get a hard, non-spongy,high pedal,
        From this behavior we can deduce a few things;
        A) That it is not spongy,indicates that there is NO air in the system.
        B) After the 4 pumps, the pedal remains hard. This indicates that there are no external leaks.
        C) As to the 4 pumps; this seems to indicate that the M/C is pumping out a lot of fluid to make it hard.

        As to A), no action is required.
        As to B) verify no external leaks
        As to C), Assuming the M/C is plumbed correctly, This has to be a fault in the rear system........ cuz the front is stuck ON. So;
        Remove the rear brakes all of it, and clamp the w/c pistons into the w/c's, so they cannot move, and the clamps cannot fall off. Now go step on the pedal. It should be hi,hard,not spongy, and on the first stroke; I mean where can the fluid go right? But if it is not high and hard, and still takes more than one pump, and there are no external leaks;......... think about it. Yes, the M/C is bypassing fluid internally, and so is defective.

        That's the best I can offer
         
        Last edited: May 11, 2020
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        • Justin Smith

          Justin Smith Well-Known Member

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          I figured Id give y'all an update. I ended up buying a new front disc brake kit for the front of the car considering I wasn't real sure what the previous owner had put on the car. most 70 B bodies have calipers mounted on the front of the rotor, mine were on the rear of the rotor. When I tried finding a new set of hoses for the set up I had, I couldn't find a single one that matched what I had. So, I got a hold of Dr. Diff, Purchased a front mounted caliper with drilled rotor brake kit, Installed it, bled the brakes and behold! I have a great pedal and car stops real well! thanks for all y'alls help!
           
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