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bench bleed master cylinder???

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by Kodiak, Apr 8, 2002.

  1. Kodiak

    Kodiak 1/2 ton status

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    Changing the master cylinder today. What is the process to bench bleed the master cylinder? Also, when bleeding the brakes, is there anything special to do with the proportioning valve?
    thanks
     
  2. skyblazer

    skyblazer 1/2 ton status

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    To bench bleed, you just need to pump the cylinder and return the fluid to the res. (most master cylinder kits have two plastic fittings and two short lengths of clear tubing to do the job). Pump the cylinder several times slowly to avoid trapping air and continue until no air bubbles are formed at the outlet. After that, mount the master cylinder and hook up the lines to the combination valve. When bleeding the wheel cylinders and/or calipers, the metering pin (in the front of the combo valve under the rubber cover) must be pressed in and held. This prevents the shuttle valve in the combo valve from sensing a fluid leak and cutting off flow to that side of the system (front or rear). From this point, bleed the brakes as normal and don't run out of fluid or you will rant and rave and throw tools and kick stuff and have to start all over again.
     
  3. Triaged

    Triaged 1/2 ton status

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    I have had good luck gravity bleeding. When I did my axle swap ALL of the fluid had leaked out of the master and all the lines. I just opened up the rear bleeders (after cleening the dirt/rust out of them) and waited till the fluid came out (it took a while...like 30min). I let it run out into a cup for a while and checked the level in the res. every few min. while the bleeders were open. When the front was done I did the front. It only took like 3min for the fluid to start flowing. I did it this way because I didn't have anyone around to help me (it was like 2am) and it worked great. I haven't tried it yet with any bubbles (just all empty like this time and to change the fluid which I do whenever I do the brakes) so I don't know if it would work in your case but it is worth a try. It is also allot easier on the master than the regular way of pumping the peddle.
     
  4. chevyracing

    chevyracing 1/2 ton status

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    don't forget to have the ends of the tubes in clean brake fluid or it will suck air back into the master cylinder.

    John
     
  5. JungleBoy

    JungleBoy 1/2 ton status

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    Be careful if you hold your BMC in a vise while bleeding - it is possible to deform the cylinder if you squeeze it, making it "junk".... but you won't know it until it is installed and you can't generate pressure with it.
     
  6. Kodiak

    Kodiak 1/2 ton status

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    I've got a question about the proportioning valve.
    On the old one on the truck, the plunger with rubber cover is fully pressed in. On the new one I have, I can pull it out some, but can't push it in any. Does it push out when the pedal is pressed? What position should that plunger be when there is no pressure?
     

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