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Im hoping someone can help me figure this out, Im boggled! I changed the brake pads on my 97 tahoe, a no brainer procedure Iv done a thousand times and now the pedal goes to the floor, Ive bled them over and over and even replaced the master cylinder, and cannot find any fluid leaking anywhere. They will pump up fine when the trucks not running but when I start it, it goes to the floor. It still stops but theres no stiff pedal it slowing makes its way to the floor. What could it be!!? Help
 

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slow to the floor sounds like a bad master cylinder to me. It's possible you got a defective MC, but kind of unlikely.

Ya got me baffled. Sorry, best I can suggest is to return the MC as defective, and try a new one.
 

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the only thing i can say is, Bleed in this order, Right Rear, Left Rear, Right Front, Left Front, twice... sometimes, you can get air in the lines when you recompress the caliper's piston back in, rarely, but does happen. i have had this happen on my 89 suburban twice. turned out in mine, it was a bad Proportioning valve.
 

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As mentioned, it is recommended that you start you bleed from the wheel farthest from the master cylinder and work your way back. It does sound like a bad mc to me. Here is some 'low pedal' info from about.com

Brake Pedal Goes To The Floor When It Is Depressed:
When the brakes are applied the pedal goes almost completely to the floor. The vehicle may not stop or needs a lot of distance to come to a stop.
Possible causes:
  1. Brake fluid is very low: Check the master cylinder fluid level and fill to the marked level.
  2. The Master Cylinder is bad: Replace master cylinder.
  3. Air in the hydraulic system: Bleed hydraulic system
  4. Large leak in brake hydraulic system: Locate and repair leak
 

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Are you working with the truck level? If not, there could be air trapped at a high spot in one or more of the wheel cylinders/calipers.
 

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Those, and I'm sure you bench bled the master cyl. first. Sometimes you get a stubborn air bubble that will not come out peacefully or easily! Also check your calipers for leaks around the pistons under the dust boots.
 
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