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is my mc blown - if not then what? (brake problems)

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by solowookie, May 15, 2002.

  1. solowookie

    solowookie 1/2 ton status

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    OK guys here are the symptoms;

    I'm driving down the road, and go to pull of the interstate & stop to realize that I can push my brake pedal in about 2" further then it usually goes, and it is soft on top of that.

    well I check fluid etc., and don't find any problems. here is the interesting part - when the vehicle is turned off (i.e. no vaccum) then if feels a lot closer to normal.

    what should I replace.
     
  2. Twiz

    Twiz 1/2 ton status

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    You need to do some further diagnostics first.

    - How common is this, Is the lower pedal, all-the-time or is it only sometimes? Does it hold and fade? or is it solid, but lower.
    - If it does fade, when and how?

    The above questions will help zero in on possible causes. We (as readers) can't really get a first hand "feel" over the net, so the most detaild discription possible can help, sometimes.

    A low, but solid pedal could be as simple as an adjustment poroblem (normaly rear drums, how is the park-brake level?)

    A pedal that will catch when "poped" than fades as the pressure is kept steady. Could be a bad master cyl, or possibly leaking wheel/calipers (usuealy masters cyls)

    A pedal that is mushy and soft all the time, could be air in the system or leaking wheel cyls/lines/hoses, ect. Or contaminated brake fluid.


    With/out "power" brakes, the pedal might feel firmer and solid, and might also feel "higher" than normal.
     
  3. solowookie

    solowookie 1/2 ton status

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    remember first of all that I have disk brakes all the way around on this;

    I can push it lower then I normally can by a couple inches, and it doesn't seem to fade more, but it isn't firm like it normally is.

    this came on all of a sudden, but I'm not sure that makes any difference.

    if I decide to change the brake fluid then what is the procedure?
     
  4. solowookie

    solowookie 1/2 ton status

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    p.s. and it is like this all the time - I don't know how hard the mc is to change, but I have one on a wrecked k5 that is sitting in my driveway.

    I'm considering changing it, and the fluid. I'm not sure how difficult that would be, but if it isn't too involved then I may just go that route anyway.
     
  5. Twiz

    Twiz 1/2 ton status

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    -K-

    First,
    Put the Blazer in drive (engine idleing) on a slight hill, slowly come to a rolling stop and keep the pedal at the same applied force.
    Does it fade, or is it steady? (very important first step, after normal "walk'round check")

    What you are checking here is,
    The master cyl has "umbrella" seals, when the pedal is pushed, the umbrella flares-out and seals the master cyl bore.
    But, as pressure remains the same, the umbrella seal relaxes and slighly bleeds off pressure, lowering or fadeing brake pedal applied height.


    Also, to check "brake pedal applied height".
    Run a tape measure from the pedal to the steering wheel and check the measurement. (you will have to judge the applied force, and figure the measurement will change with pressure)
    A gradual change should be evident.
     
  6. solowookie

    solowookie 1/2 ton status

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    got a little hill about 50 yards from my house - it stays firm - doesn't bleed off. height seems to be the same.
     
  7. woody9

    woody9 1/2 ton status

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    Jeff,
    Sounds like yours has been parked next to mine and now they've both got it! SInce i put the 4 wheel disc on, the pedal height seems VERY low.
    bled & bled & bled & bled the rears, and still not quite where I like it. I have it scheduled for a 'power bleed' at a local shop shortly. He thinks I still have some air in my lines.
    I also have a m/c I think I'll put in before he does that, and will also bleed my fronts (if i can get those dang bleeder screws loose! else it's new calipers for the front as well!!)....
    With the age of ours, seems a fluid swap would be good preventative maint as well.
    I'll keep u posted on what gives with mine.
     
  8. Twiz

    Twiz 1/2 ton status

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    It's been like this since the disk-brake swap?
    I thought you changed to a higher-rated Master cyl, when you did the conversion.

    It sounds like you have a good master cyl, it's just not displaceing enough fluid to cover the extra volume needed by rear calipers.

    Swaping Master cyls of the same style, probably won't have any effect.
     
  9. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Check all your lines and calipers for leaks. My concern here is the master for a system with rear dums doesn't bias the pressure correctly. It takes more line pressure to activate a Disc then it does a drum. It also takes more fluid.
    Did the Brake light in the dash come on at the same time (sure it's working?) It's possible that the difference in pressure and volume has freaked out the proportioning valve.
    As far as changing the master....don't bother with a used one. get one from a parts place. A rebuilt master fron Auto zone is $13 with a core and a NEW master is $35. Hardly worth the time to remove a used one that may or may not be good.
     
  10. HarryH3

    HarryH3 1 ton status Author

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    Do your rear calipers have parking brakes? If so, you have to get all the free play out of the calipers first. The pads need to be snugged up to the rotors, just before they drag. Otherwise, the pedal will go way down due to the amount of fluid that must be displaced before the rear pads make good contact with the rotor surface.
     
  11. solowookie

    solowookie 1/2 ton status

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    no it has NOT been like this since the conversion; yes I had a very hard time getting all the air out initially, but this just started all of a sudden.
     
  12. solowookie

    solowookie 1/2 ton status

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    let me go over some of this again; rear disk conversion was done about this time last year. all the bugs were worked out if the system, and it was working perfectly, but I do only have about 6,000 on the brakes. when I did the rear conversion I also replaced rotors, calipers, and pads front & rear so the entire break system (excluding the mc of course) has 6,000 miles on it.

    pedal height / pressure has been perfect for 6,000 miles. it took a lot of bleeding the lines in the beginning to get all the air out because it was pulling to one side real bad; however, that problem has been taken care of for a looooong time.

    I was going from my hometime on Lyman to Evanston (about 40 miles) to get my new wheels swapped on, and everything was perfect when I left. when I got off the freeway, and went to stop is when I noticed the problem / difference.

    there are no rubber lines on this system; stainless steel braded lines up front, above the pumpkin in the rear, and short stainless steel lines that go to the rear calipers also.
     
  13. solowookie

    solowookie 1/2 ton status

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    rear calipers have e-brake arms, but they have never been hooked up; I need to measure out how long the cables need to be.
     
  14. HarryH3

    HarryH3 1 ton status Author

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    Cycle the e-brake levers a couple of times. They have a mechanism that is designed to keep the slop out of the calipers as the pads wear. If that doesn't fix it, then it may indeed be time for a new master cylinder. /forums/images/icons/frown.gif
     
  15. solowookie

    solowookie 1/2 ton status

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    I'll have to do that - thanks for the advice...
     
  16. solowookie

    solowookie 1/2 ton status

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    ok - here's the answer to the great mystery - that black umbrella piece was having some serious problems! to make a long story short it was bad, and so all I did was swap lids from that wrecked k5, and Darth Blazer, topped of the fluids, and problem solved!
     
  17. solowookie

    solowookie 1/2 ton status

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    ok guys - looks like I jumped the gun a little too quick /forums/images/icons/tongue.gif.

    well - pedal is getting softer all the time - and is progressively getting worse. umbrella etc. is OK, and fluid levels are good. I had my wife pump the brakes pretty good while I watched, and there doesn't seem to be any leaks anywhere, and fluid level isn't changing.

    right now - pumping it does seem to make it firm up a bit.
     
  18. Twiz

    Twiz 1/2 ton status

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    The black rubber gasket/diaphram under the cap was deformed and swullen? -Correct? (looks like it was too large for the lid, or doesn't fit properly, feels kinda-extra soft)

    Thats usualy a bad sign, something other than DOT3 brake fluid was used, or the fluid was contaminated in some-other way.
    The rubber seals and gaskets used in the brake system are very sensetive to oils, when they come in contact with it - they swell, bloat and deform.

    If this is the case.
    Do Not Bleed it! (yet)
    Compress all four caliper pistons, all at once (wedge something between the caliper and rotor to hold the piston in place as you compress the others)
    If you are lucky, this will hopefully back flush the contaminated fluid out of the master cyl, and hopefully minimize the damage done to just the rubber parts that are up-stream.
    Remove the master cyl, disassemble it and inspect. Check the rubber umbrella seals inside the master cyl for signs of contamination (swollen/soft/deformed) also check the cyl bore for scratches or marks.

    If the seals do show signs of contamination, replace the master cyl. (and inspect for further damage)

    It is entirely possible that every-single rubber part on the brake system may need to be replaced. It depends on how far the contaminated fluid traveled though the brake system. To be safe, it *should all be replace*, even if it does not show signs of being contaminated.
     
  19. solowookie

    solowookie 1/2 ton status

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    we won't talk any names, but some moron put brake fluid inside that piece instead of removing it with the lid... /forums/images/icons/tongue.gif LOL I think it is salvagable, but it wasn't in place where it should be long enough that I'm not real confident it will hold the shape it should.
     
  20. Twiz

    Twiz 1/2 ton status

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    Ahh, I see./forums/images/icons/smile.gif
    Do you think it might have just sucked some air into the system then?
     

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