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Technical disc brake swap issue..still got a soft pedal

Discussion in 'The Hokey Ass Message Board' started by Vincentnova, Dec 2, 2018.

  1. Vincentnova
    Joined: Nov 6, 2015
    Posts: 62

    Vincentnova

    I did a front disc brake swap ( kit came with everything even brake lines)
    all new parts.
    master cylinder has been bench bleed more than one time with a big syringe made for it
    rod from booster to MC has been adjusted
    I bled the system I dont know hoooooow many times...
    I bled the system old fashion way , gravity ,2 persons then ..with a pressure tool
    i banged as well with a rubber hammer on calipers
    i even bled the caliper out of there bracket bleeder facing the ceiling to make sure....
    there is no leaks
    yes the caliper are mounted bleeder valve on top
    only new fluid has been used
    now--- engine off I have a mid hard/spongy pedal and when engine is on pedal goes to the floor


    I end up by pinching the hoses to see if i could located the issue
    there is 3 hoses
    if I pinch the rear one , nothing change
    if I pinch the front one , both at same time , or one each time , I get an hard pedal.........

    any idee ????
     
  2. rockable
    Joined: Dec 21, 2009
    Posts: 2,636

    rockable
    ALLIANCE MEMBER

    Observe your calipers very closely with a helper applying the brakes. See if one or both of them are "sucking back". This will cause the problem you describe. Usually caused because there is no vent to the MC.
     
  3. F&J
    Joined: Apr 5, 2007
    Posts: 13,213

    F&J
    ALLIANCE MEMBER

    "normally", on a factory built OEM car, this means air in the system.

    What type of proportioning valve are you using?

    next question is do you have real good flow from front and rear bleeders when one person holds the pedal down, and you open a bleeder?

    .
     
    porknbeaner and Fordors like this.

  4. I will have to say the same.... or did even get one for disc brakes?
     
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  5. Vincentnova
    Joined: Nov 6, 2015
    Posts: 62

    Vincentnova

    i have a proportioning valve for front disc brake/rear drum .
    yes i have equal and good flow coming out of each bleeder at all corners when someone press the pedal
     
  6. F&J
    Joined: Apr 5, 2007
    Posts: 13,213

    F&J
    ALLIANCE MEMBER

    I asked...as some OEM valves need to have the spool (that runs the warning light) to be held at "centered position", to get proper flow to both circuits.
     
  7. Patrick Crumley
    Joined: Sep 12, 2018
    Posts: 21

    Patrick Crumley

    I had this problem on an OT El Camino. Bought one of the cheap kits off epay that came with everything in one kit. Everything was new from front to back, tried everything to get a pedal with the exact problem you are describing. Finally ended up putting 2# residual valves on the front lines. Problem solved for few thousand miles now. Not sure if it's the right thing to but it helped me.

    Sent from my moto z3 using The H.A.M.B. mobile app
     
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  8. Tooth
    Joined: Nov 5, 2016
    Posts: 119

    Tooth
    Member
    from Missouri

    Don't know if you got your spongy brake pedal problem fixed yet but Speedway Motors makes a power bleeder that looks like a spray bottle that has a pressure gauge a hose and a plate on it. The plate clamps on the master cylinder and then you can fill the canister with brake fluid and pressurize the system. Up to 15 psi, then you can go to the right rear and Bleed it first and get all the air out of it then go to the left rear right front and left front maintain at least 10 psi of pressure on the gauge. The proportioning valve piston can be held in place with a tool that looks like a little white plug with a tip on it. You do not have to press the brake pedal when you use the power bleeder system it's 80 bucks. You unscrew the plug out of the center of the proportioning valve for the wire hooks to to warn you that you have a brake problem and you put the little plastic plug with the tip in it to hold the piston in place if you're having someone pump the brakes. I put a manual proportioning valve in my rear brakes and nothing in the front, you can adjust the rear down to where it is 60-40 50-50 ect. Good luck!
     
    lilmann likes this.
  9. Vincentnova
    Joined: Nov 6, 2015
    Posts: 62

    Vincentnova

    yes i do have that tool that pressure bleeding.the same one you talked about with the plate for the MC
    and believe a used it a looooooooot loool
    and i also have the little plug tool for the proprtioning vavle to keep in place the valve , and i have it installed since the beginning ( i would never bleed without that...ive been there before lol)
     
  10. Vincentnova
    Joined: Nov 6, 2015
    Posts: 62

    Vincentnova


    yes me too...its the cheapest complete kit i could find on ebay.... but usually residual valves are used when MC is under the floor no? what kind did you use?
     
  11. Patrick Crumley
    Joined: Sep 12, 2018
    Posts: 21

    Patrick Crumley

    Wilwood 2# blue ones. Bought them from a local shop.

    Sent from my moto z3 using The H.A.M.B. mobile app
     
  12. Garpo
    Joined: Jul 16, 2016
    Posts: 152

    Garpo

    I have encountered calipers that have the bleed screw at or near the bottom. Screw connects to the chamber via a drilling. If mounted "wrong side", this brings the bleeder to the top, and the drilled port is on the bottom of the chamber, leaving an air space above and impossible to bleed.
    With most of these, the track of the drilling can be seen as a raised ridge on the caliper body leading to the bleed screw..
    Trying to remember what these were fitted to - - might be Mopar?
    Garpo
     
  13. Vincentnova
    Joined: Nov 6, 2015
    Posts: 62

    Vincentnova

    ok than

    ok thanks just order it..will let you guys know
     
  14. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,284

    Kerrynzl
    Member

    If you have GM callipers , chances are you have low drag callipers [these retract in the bores more than normal callipers]
    Low drag callipers need a stepped M/C [quick uptake M/C] or this problem will never go away.

    Nothing is wrong with your components, they are just mismatched.
    Low drag callipers can be ID'd with metric bleed screws.

    You can use a feeler gauge to check the clearance between pads and disc [they retract about 20-25 thou vs 5 thou for normal callipers]
     
    Last edited: Dec 3, 2018
    weps, ottoman, Atwater Mike and 4 others like this.
  15. Patrick Crumley
    Joined: Sep 12, 2018
    Posts: 21

    Patrick Crumley

    I looked everywhere for this kind of answer to my problem. I tried a new MC of course it was the same style as the one that came with the kit. Finally tried the residual valves as I was thinking the MC didn't have enough volume to keep the calipers full. I knew there was no air in the system. Your post makes perfect sense. I guess that is what I get for buying a cheap kit..

    Sent from my moto z3 using The H.A.M.B. mobile app
     
  16. Vincentnova
    Joined: Nov 6, 2015
    Posts: 62

    Vincentnova


    thanks a lot very help full answer
    I think you right after reading your answer I just find this online on different forum ' test low drag caliper.
    im gonna try that as well

    https://www.hotrodders.com/forum/test-low-drag-calipers-158523.html
     
  17. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,284

    Kerrynzl
    Member

    You really need to ID what you have.

    A quick uptake M/C [stepped bore] is the best solution.

    Below shows the difference in the callipers. Most parts counter jockeys don't even know the difference
    And it is common for the wrong type to be exchanged with rebuilders.
    square seals.jpg
     
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  18. Vincentnova
    Joined: Nov 6, 2015
    Posts: 62

    Vincentnova

    i tried a MC from a 1972 chevelle for disc brake front and rear drum. with the bigger bowl in front for disc.
    i think those MC are step bore ? no?
    if no ?what make a MC a step bore how tell ?

    as well i went auto parts stor an got different caliper from a 78 malibu , since i find online that gm started low drag caliper in 1980

    also i find out with the front wheels on car i have a negative camber????? before with drum the front wheels were straight...i dont get it either about that..
     
  19. Vincentnova
    Joined: Nov 6, 2015
    Posts: 62

    Vincentnova

    forgat to mention previously , even with the different master or 78 caliper i still have same issue
     
  20. gimpyshotrods
    Joined: May 20, 2009
    Posts: 16,252

    gimpyshotrods
    Member

    That is presuming that the parts house gave you the correct caliper, and that the rebuilder gave the parts house the correct caliper.

    A 1972 Chevelle master cylinder is not a steeped bore master cylinder. Two different bowl sizes does not indicates stepped bore.

    Disc brake master cylinders have a bigger bowl for the disc circuit, because, as pads wear, and the piston moves out, more fluid is needed to take up the space.

    This is why the ubiquitous, generic, "Corvette" style master cylinder, which is a 4-wheel-disc master cylinder, can be used on 4-wheel-disc, disc/drum, or even 4-wheel-drum applications. When not required, the extra fluid is simply not used.

    Stepped-bore master cylinders did not come into play until the low-drag calipers did, in most cases.
     
    Kan Kustom likes this.
  21. LM14
    Joined: Dec 18, 2009
    Posts: 1,760

    LM14
    Member
    from Iowa

    What brake fluid?
     
  22. gimpyshotrods
    Joined: May 20, 2009
    Posts: 16,252

    gimpyshotrods
    Member

  23. Vincentnova
    Joined: Nov 6, 2015
    Posts: 62

    Vincentnova

    i used DOT 3
     
  24. Vincentnova
    Joined: Nov 6, 2015
    Posts: 62

    Vincentnova


    yes the kit came with the generic, "Corvette" style master cylinder
     
  25. gimpyshotrods
    Joined: May 20, 2009
    Posts: 16,252

    gimpyshotrods
    Member

    That is a mismatch for a low-drag caliper. Please follow the instructions provided by @Kerrynzl to determine if you have low-drag calipers.
     
  26. Vincentnova
    Joined: Nov 6, 2015
    Posts: 62

    Vincentnova

     
  27. gimpyshotrods
    Joined: May 20, 2009
    Posts: 16,252

    gimpyshotrods
    Member

    Also, if you have any drum brakes in the system, that circuit must have a 10lb. residual valve, to keep the seals pressurized, to prevent sucking air back in, irrespective of where the master cylinder is mounted.

    Also, your rear shoe drag must be set properly. Rear shoe drag has a big effect on pedal-feel.

    "How do you physically check if your calipers are low drag? To check if your caliper is low drag perform the following test. Obtain a pair of brake hose clamps at an auto parts store. Try the pedal as it is and then clamp off the two front hoses. If the pedal returns and is high and firm chances are you have low drag calipers. Be aware that this same sympton will occur if there is still air in the caliper or the bleeder screw that lets the air out during bleeding is not directly on top in the 12:00 position facing up."
    Source: http://www.idmsvcs.com/2vmod/brakesystem/hydraulic/disassembly/calipers/pdfs/mpbrakes_lowdrag.pdf

    Sure sounds a whole lot like:
     
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  28. s55mercury66
    Joined: Jul 6, 2009
    Posts: 3,719

    s55mercury66
    Member
    from SW Wyoming

    If you are seeing a camber difference, that indicates there is slop in your wheel bearing arrangement, and that is forcing your pistons farther into the calipers than they are designed for, and you are taking up that difference when you step on the brake pedal.
     
  29. gimpyshotrods
    Joined: May 20, 2009
    Posts: 16,252

    gimpyshotrods
    Member

    Non-stepped bore master cylinder, for a GM metric caliper:
    [​IMG]

    Stepped bore master cylinder, for a GM metric caliper:
    [​IMG]
     
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  30. Vincentnova
    Joined: Nov 6, 2015
    Posts: 62

    Vincentnova

    here a picture of my camber as well.
    was not like that before 20181209_114259 (2).jpg fore
     

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