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sinking brake pedal - master cylinder?

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    #1

    sinking brake pedal - master cylinder?

    The vic has always had garbage brake feel, and has slowly been getting an issue where the brake pedal sinks, slowly, as you come to a stop. It never gets all the way to the floor, or really even worryingly close. It feels like a leaking brake line at first, but after sinking maybe half an inch over 5-10 seconds, the sinking stops. It's just enough that with light braking you have to continually move your foot down with it.

    Brakes themselves (all 4 corners and the flex lines) are in known good condition, and the fronts have been bled recently. The steel lines are newer and the car has been panic-stopped dozens of times since I got it. has not lost any brake fluid as far as I can tell in about 5,000 miles of driving since I got it.

    The only things I can think to check is I haven't inspected the driver's side wheel cylinder closely and haven't bled the rear brakes.


    Is a bad master cylinder a good guess? anything else to check?

    85 4 door 351 Civi Crown Victoria - Summer daily driver, sleeper in the making, and wildly inappropriate autocross machine
    160KMs 600cfm holley, shorty headers, 2.5" catted exhaust, 255/295 tires, cop shocks, cop swaybars, underdrive pulley, 2.73L gears.
    waiting for install: 3.27's, Poly bushings, boxed rear arms, 2500 stall converter, ported e7's, etc

    06 Mazda 3 hatch 2.3L 5AT (winter beater that cost more than my summer car)
    Tags: master cylinder
    #2
    Wheel cylinders or the MC is a good bet. Follow the fluid. If you see signs of fluid between the MC and the booster, you have your answer.
    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC grandpa spec white and cranberry

    1984 Lincoln Continental TurboDiesel - rolls coal

    Originally posted by phayzer5
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

    Everything looks like voodoo if you don't understand how it works

    Comment

      #3
      I've never seen any fluid loss. I've kept track as closesly as I can and found no dampness anywhere and no fluid level loss in the reservoir.

      85 4 door 351 Civi Crown Victoria - Summer daily driver, sleeper in the making, and wildly inappropriate autocross machine
      160KMs 600cfm holley, shorty headers, 2.5" catted exhaust, 255/295 tires, cop shocks, cop swaybars, underdrive pulley, 2.73L gears.
      waiting for install: 3.27's, Poly bushings, boxed rear arms, 2500 stall converter, ported e7's, etc

      06 Mazda 3 hatch 2.3L 5AT (winter beater that cost more than my summer car)

      Comment

        #4
        If the seal(s) inside the master cylinder are bad it can have an internal bypass type leak that is undetectable with external fluid... it sounds like a MC problem to me. A rebuild kit isn't that much...however a whole new unit isn't that much more really.

        But definitely examine the rear brakes closely especially that wheel cylinder first...if there are no issues back there then I'd say it's the MC
        Click image for larger version Name: master-cylinder.gif Views: 1 Size: 36.3 KB ID: 1265455
        Last edited by Brown_Muscle; 07-27-2011, 06:45 PM.
        -Phil

        sigpic

        +1982 Ford LTD-S Police Car. Built 351w, Trickflow 11R 190 Heads, Holley Sniper EFI, RPM Intake+ Hyperspark dizzy, WR-AOD, Full exhaust headers to tails. 3.27 Trac-Lok Rear. Aluminum Police Driveshaft. Speedway Springs+Bilstein Shocks, Intermediate Brakes, HPP Steering Box.

        +2003 Acura CL Type S 6-speed

        Comment

          #5
          that's sort of what I was thinking. It feels like the bore is scored and leaking at the start of travel, but as the pedal sinks a bit it seals better.

          85 4 door 351 Civi Crown Victoria - Summer daily driver, sleeper in the making, and wildly inappropriate autocross machine
          160KMs 600cfm holley, shorty headers, 2.5" catted exhaust, 255/295 tires, cop shocks, cop swaybars, underdrive pulley, 2.73L gears.
          waiting for install: 3.27's, Poly bushings, boxed rear arms, 2500 stall converter, ported e7's, etc

          06 Mazda 3 hatch 2.3L 5AT (winter beater that cost more than my summer car)

          Comment

            #6
            I wouldn't bother rebuilding these, the remanufactured ones are cheap enough and they're guaranteed to work. A rebuild kit doesn't guarantee your work I agree with an internal Master cylinder leak.

            Comment

              #7
              I'll definetely be buying new. I'm making up an order for RockAuto and was trying to decide if a new MC should be part of it, and I think this thread decided in the positive. That thing is an eyesore anyway.

              85 4 door 351 Civi Crown Victoria - Summer daily driver, sleeper in the making, and wildly inappropriate autocross machine
              160KMs 600cfm holley, shorty headers, 2.5" catted exhaust, 255/295 tires, cop shocks, cop swaybars, underdrive pulley, 2.73L gears.
              waiting for install: 3.27's, Poly bushings, boxed rear arms, 2500 stall converter, ported e7's, etc

              06 Mazda 3 hatch 2.3L 5AT (winter beater that cost more than my summer car)

              Comment

                #8
                Originally posted by johnunit View Post
                I'll definetely be buying new. I'm making up an order for RockAuto and was trying to decide if a new MC should be part of it, and I think this thread decided in the positive. That thing is an eyesore anyway.
                You gonna paint it?

                Comment

                  #9
                  Originally posted by 86VickyLX View Post
                  You gonna paint it?
                  the new one? depends on how it looks I guess.

                  I already tried painting the old one a couple times and I guess I fail at getting all the brake fluid off before painting. it's got about 4 layers of paint peeling off the reservoir.

                  85 4 door 351 Civi Crown Victoria - Summer daily driver, sleeper in the making, and wildly inappropriate autocross machine
                  160KMs 600cfm holley, shorty headers, 2.5" catted exhaust, 255/295 tires, cop shocks, cop swaybars, underdrive pulley, 2.73L gears.
                  waiting for install: 3.27's, Poly bushings, boxed rear arms, 2500 stall converter, ported e7's, etc

                  06 Mazda 3 hatch 2.3L 5AT (winter beater that cost more than my summer car)

                  Comment

                    #10
                    Originally posted by johnunit View Post
                    the new one? depends on how it looks I guess.

                    I already tried painting the old one a couple times and I guess I fail at getting all the brake fluid off before painting. it's got about 4 layers of paint peeling off the reservoir.
                    Clear coat it. Shiny cast iron MC.

                    Comment

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