brake pedal sinking

1993 CHEVROLET SUBURBAN
2WD • AUTOMATIC
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CZR1760
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I have the SUV listed above it is a 1500 with over three hundred thousand miles. I recently started having problems with the brake pedal sinking to the floor. This problem seems to be more noticeable when I am stopped (at a traffic light) and holding. I first tried to bleed the brakes, but this had no effect. I changed out the master cylinder (re-manufactured), bled the brakes and still have the same problem. I also had to install a new front caliper, because the bleeder bolt would not come out of the old one. The truck seems to stop okay, but has a soft pedal. If I press on the brake pedal quickly, I can hear a "woosh" noise. I do not know if I got a bad master unit or if I have a bad vacuum booster as well. Any help would be appreciated. Thanks.
Aug 5, 2011 at 5:20 PM
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DOCFIXIT
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Rebuilt masters are no good I would get a new one.

Here is a guide to help you:

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

Please let us know what you find.

Cheers
Mar 15, 2021 at 11:47 AM
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CZR1760
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I bought it from O'Rieley's so I can just take it back. I did a good bench bleed on it before installing it. I read that I could clamp off the brake lines and then see if the pedal got hard, just to make sure that it was the master. It is really easy to pull off, so I will probably just swap them out. Should I suspect anything with the vacuum or just leave it alone? Thanks.
Mar 15, 2021 at 11:47 AM
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DOCFIXIT
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If rear wheel cylinders are not leaking or sucking air. Thing to do is with small screwdriver pull boot back if fluid comes out then replace wheel cylinders first. And bench bleed is way to go.
Mar 15, 2021 at 11:47 AM
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CZR1760
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I swapped out the re-manufactured master for a "new" one. Bench bled, bled out the lines and still have the same problem. While the truck is running, the pedal does not seem to fully engage until about half way. If I apply pressure very quickly the pedal seems like it has correct feel (although you will forward if going too fast). If I apply slowly the pedal is down about half stroke and continues almost to the floor when stopped. The pedal travels so far down, that if I push slightly further than required to stay stopped, the "brake" light comes on the dashboard. At that point the pedal is completely stroked out. Where to go from here? Thanks.
Mar 15, 2021 at 11:47 AM
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DOCFIXIT
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Did you check rears?
Mar 15, 2021 at 11:47 AM
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CZR1760
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Checked both rear wheel cylinders, no leaks. I also gravity bled the system as well. I did get some air out, but still the same result, sinking pedal. Very frustrating.
Mar 15, 2021 at 11:47 AM
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DOCFIXIT
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Suspect air in BPMV see diagram this needs to be opened with a scanner when bleeding the brakes.
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Mar 15, 2021 at 11:47 AM
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CZR1760
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Would I be able to bleed any air out of the BPMV or am I stuck taking it to a shop at this point? I will need to double check when I get home, but I believe that it has four wheel ABS. Again it is a 1993 C1500 Suburban. Looking at the diagrams that you provided, it does not resemble RWAL control module. Is there any way to manually bleed BPMV, with or without an assistant? I have invested enough time that I would like to do this myself, if possible. Thanks.
Mar 15, 2021 at 11:47 AM
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DOCFIXIT
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In first figure open right bleeder screw 1/4 to 1/2 turn slowly depress pedal hold now in figure two open right bleeder until air is expelled. close repeat until all air out.Then close it an internal right bleeder. do same for left.
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Mar 15, 2021 at 11:47 AM
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GLENN WHEELER
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I had this problem and this fixed it thanks!
Mar 15, 2021 at 11:47 AM
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DOCFIXIT
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Good to hear. Happy Motoring!
Mar 15, 2021 at 11:47 AM