now what???? (was power booster)

ok, so we replaced the master cylinder, we bench-bled it as directed, then bled the brakes. started it up, and instead of a full-floor mush pedal as before it
was now like the engine was off, a hard pedal with little grab requiring a very hard push to stop the truck, and the engine revved slightly when pushing the brakes. obvious vacuum problem there and the hard pedal is indicative of power booster failure. soooooo, we replace that as well, and we still have a mushy pedal. although mushy, it does grab hard at some point and we can get the brakes to lock up, but it aint at all right.
any other ideas on why we have still have mush? when bleeding, starting with the right rear, how many pumps or how much volume should you bleed to feel sure that the entire length is flushed? we are using the little magnetic bottle bleed kit from auto zone.
david
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Dave,
Bleed them out the ol' fashioned way (without your bleed kit or vacuum pump). Two slow pumps, hold pedal, open bleeder, when pedal hits the floor close the bleeder and pump em' up again. I like to put a clear hose on the bleeder valve so I can "see" what's coming out, including little tiny bubbles that you might miss using another method. When there are no bubbles, it's bled. It really varies as to how many pumps/how much volume is required. Seeing as how you opened the lines at the very beginning of the brake circuit, it's gunna take awhile, as the air at the beginning of the line needs to make its way all the way to the other end.
If it's bled correctly and you still have a low/mushy pedal, check the tightness of the rear brakes, as out-of adjustement rears will cause a low, mushy pedal. Year, make and model?
Doc

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snipped.....

thanks doc - its a '96 chevy z-71 w/350
oh yeah, and its red .. ;-)
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Oh, the red ones require a special tool to bleed the brakes.......................that must be why you're having such difficulty...............he he he he.
Doc

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Don't believe him, he did the same work to his C1500 and it's still all mushy..
GMC Gremlin

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I think I've been around on this ng long enough for people to know my advise is trustworthy. You on the other hand have not, and are currently batting around .200 per way of "advise."
If you were someone else on this ng, who would you listen to? :-)
Doc
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arrrrrrrrrrrrrrgh
ok, pumped the brake 4 times slowly, then held - it was at about 3/4 down - port (right rear) opened and i felt it go to the floorboard and then closed the port. repeat. same thing - and bubbles were noted. however, from then on - about 7 repetitions, the pedal went all the way to the floor before opening the port each time - but fluid still came out when opening the port, just not much. no more bubbles from the 4th time to the 7th so we moved to the left rear - -same thing - the pedal went all the way to the floor before opening the port, but fluid still came out and there were bubbles the first 2 or 3 tries, but not the last 4 or 5. did all 4 the same way, still all the way to the floor.
certainly this would be the exact symptoms one would expect if there were a leak somewhere, but after all that i would expect the leak would be very evident. can it leak within the abs and not be noticeable????
tomorrow morn we will check the rear brakes, but i cant imagine them being so loose that they would absorb all the hydraulic effort. should we maybe try the vacuum method of bleeding and run about 5 gallons of fluid through each line to make dawggone certain it is entirely flushed???
this is aggravating, on a 1 to 10 scale i would say i can do up to 7 or so skill-level work, so i dont think we are complete idiots - . but then again i am a surfer from texas so who knows?? ;-)
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Inline...................................

Hmmmm, not good.

No, not really. The ABS system is in bypass mode when it's not running, so it's unlikely that any air would have made it's way into the ABS hydraulics. As for the ABS leaking, I've never heard of that before.

If you're not getting any hydraulic pressure, this effort might be in vain.

He he. Try this. Open the Master cylinder reservoir and watch the bottom of the reservior as someonle else VERY SLOWLY depresses the pedal. If they press it too fast you'll get wet! You should see two mini-geysers of fluid coming up, one from each reservoir. Are there any bubbles in this stream of fluid? After pumping them up a few times is the master cylinder fluid foamy?
Doc
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saw the "geysers, and no, no foaminess or bubbles.
wait a minute, maybe i AM an idiot after all. is this supposed to be done with the engine running or not?
damn, that was a hard question to ask. good thing embarrassment doesnt translate well into ascii... ;-)
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You'll want to bleed them with the engine off. How have you been doing it?
Doc
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um, with it - um - off, of course.
yeah, thats the ticket... ;-)
whats the difference?
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7 reps...took me about 40 reps to get the all of the air out starting from the the passenger rear when i replaced my master (bled it the old fashioned 2 man way that doc described)...the solid stream of brake fluid could be the old fluid still in the brake line with the residual air from the master still traveling through the lines (as i experienced)...i was going to give up after the 20th push but continued on and it eventually blew out more air...used half a quart of fluid)...top shape after...

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thanks. i have a tv in the garage so i can do it during a game, poor son though, he's the one on the ground... ;-)
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I know it's too late now, but it's always a good idea to bleed the tubes at the master cyl FIRST, after a m/c replacement. Then bleed at the wheels to flush the old fluid (and contaminates) out of the system. H
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Yep, I've actually had this to be all the bleeding required, of course beside the master cylinder before installing it. Not every time but about 50% of the time all of the air is gone when bleed at the master cylinder lines.
Brian
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