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speedyal
Novice
Feb 14, 2007, 10:02 AM
Post #1 of 7
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Sinking brake pedal
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'95 Jimmy 2WD. Can the ABS or combination valve cause a sinking pedal? Not a mushy pedal, no air trapped, no leaks, fluid level is fine and master cylinder is brand new. The shop is trying to tell me GM ABS units can cause this. I don't see how. Thanks!
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way2old
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Feb 14, 2007, 2:12 PM
Post #2 of 7
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Re: Sinking brake pedal
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Yes the ABS unit an cause the pedal to sink. I have been fooled a few times by the symptoms, especially if it is the one bolted close to the master cylinder. The unit will stick open and allow brake pressure to go through the ABS unit and not bypass it as it should under normal braking.
Being way2old is why I need help from younger minds
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speedyal
Novice
Feb 14, 2007, 2:18 PM
Post #3 of 7
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Re: Sinking brake pedal
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Thanks. Help me understand. Go thru it to....where? As longs as the master cylinder is making pressure and pads/shoes are hitting discs/drums, how does the fluid return to the reservoir?
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way2old
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Feb 14, 2007, 2:28 PM
Post #4 of 7
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Re: Sinking brake pedal
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When the ABS is not engaged, the fluid bypasses the ABS unit. When ABS is engaged, it takes more fluid to energize the ABS because of the extra area that the fluid needs to displace. So if the ABS is not closed off, the fluid runs through the ABS and back to the master cylinder. It just takes more fluid to do it and the edal will slowly sink just like a by-passing master cylinder. The fluid does not return to the reservoir until the pressure is released from the piston in master cylinder. What you are doin, in essence, is running the ABS system without energizing the ABS.
Being way2old is why I need help from younger minds
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speedyal
Novice
Feb 14, 2007, 3:23 PM
Post #5 of 7
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Re: Sinking brake pedal
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Okay, but I can sink it to the floor, lift off and sink it back down 50 times. Eventually I would pressurize that ABS, wouldn't I?
Not questioning, just trying to learn. That's why we need older minds.....
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way2old
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Feb 14, 2007, 6:08 PM
Post #6 of 7
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Re: Sinking brake pedal
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See if this site helps any. http://www.autozone.com/...0900823d8018caba.jsp I don't know how much of the hydraulics it goes through, but it will give you the basics as to how the ABS works. There re 2 systems that may be on your truck, RWAL(Rear Wheel Anti Lock), ot the full 4 wheel ABS system. Good luck and i will see if I can think of a good way to explain the hydraulic system on your truck. Good luck.
Hey Dan. You do a lot more Chebbys than I do. Anything to add?
Being way2old is why I need help from younger minds
(This post was edited by way2old on Feb 14, 2007, 6:17 PM)
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speedyal
Novice
Feb 14, 2007, 7:10 PM
Post #7 of 7
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Re: Sinking brake pedal
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It's a 2WD, so it's 4 wheel ABS. Interesting schematic, but I don't think there's enough detail to answer my question. I'll definitely keep it for future reference. I just can't figure out how the system can dump pressure if the MC is good and it's not squirting out somewhere. One pedal pump, maybe 2 or 3, sure, but multiple pumps has just GOT to be a bad MC, right?
Shoulda kept my '67 Mustang. No mysteries there!
The best thing about these problems is they're so hard to figure out. The easy ones are no fun.
Many thanks for your help and interest!
Al
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