Question about 1991 GMC C1500

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Spongie brakes i recently changed out the master cylinder,bled fuild out put in new fluil in whole system.bled out all air even gravity bled whole system,brakes are still not what they should be.the pedel drops a little over half way before they start to work.and then they slowly drop a bit before stopping,ive checked all wheel cylinders for leaks and all lines,all seems to be ok. please help! 91 gmc 1/2 ton two wheel drive with a 350

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  • larry creason Jan 28, 2009

    done all of the above still half pedel

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  • Master
  • 1,214 Answers

Gravity bleeding doesnt usually work real well.

Get a clear jar of some type, and a piece of 1/4 inch vacuum hose about 18 inches long. Put about 2 inches of new brake fluid in the container.

Start at the passenger rear wheel, losen the bleeder, slide the hose tightly over the open bleeder, the other end in the jar where you can see the hose end.
Fill master cylinder reservoir.

Pump brake no more than 4 times, watching the hose end in the jar for bubbles. After 4 pumps, check reservoir, refill. Pump 4 more times. Watch for bubbles. Check reservoir, pump 4 more times. If you see bubbles at any time, then keep going through the above steps until you stop seeing bubbles.

Go to rear wheel, tighten bleeder. Go to driver rear wheel, and do the same thing, although this time it won't take but 4-6 pumps to purge any air.

Go to passenger front, repeat.
Go to driver front, repeat.

Do not recycle any of the fluid.

Posted on Jan 28, 2009

  • Mark Egan Jan 28, 2009

    If your still spongy, then you have not gotten all of the air out of the system. It's as simple as that.

    You need to be sure you have pumped enough fluid through the line that you've pushed all the air out. The nice thing about the jar method is you can see dirty fluid come through, which is an indication that you have not pumped enough to push the bubble through.


  • Mark Egan Jan 28, 2009

    And half pedal could be the yoke adjustment on the linkage rod between the brake pedal arm and the master cylinder.

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