Sinking Brake Pedal

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  #1  
Old 12-02-11, 09:06 AM
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Sinking Brake Pedal

I just put the 3rd master cylinder in about the last 2 years in my wife's '93 Chevy S-10 Blazer. I have NO leaks at wheel cylinders, I bench bled the laster cylinder & bled all 4 wheel cylinders intil no air & clear fluid. The brakes work but when at a stop, the pedal will very slowly sink & the truck will start inching forward again. Pumping the brakes will get pedal back & it will stay stopped. What gives here? Am I getting multiple bad master cylinders? I've been working on brakes for years & never had this anomaly............

TexasFire
 
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  #2  
Old 12-02-11, 05:30 PM
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Are you buying rebuilt cyls? They have a dismall sucess rate. Try a new one.
 
  #3  
Old 01-08-12, 07:57 AM
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Could the ABS unit in this truck (93 S-10 Blazer) have anytrhing to do with this? My wife advised that the ABS is acting up, coming on once in a while in braking situations where it used to not. I know nothing of the ABS block near the master cylinder & not sure if there's anything user servicable in it or not.
If my pedal is sinking, then that pressure is going somewhere. I am not leaking fluid & this is my 3rd master cylinder. Sure, I could have bad luck on 3 bad units but I think that is unlikely. My wife mentioned the ABS thing before but I discounted it. Now I'm beginning to wonder.........

TexasFire
 
  #4  
Old 01-08-12, 09:19 AM
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You might be able to pull the fuse for the ABS as a temporary test.
 
  #5  
Old 01-08-12, 02:52 PM
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the only thing that can cause that is fluid bypassing somewhere, either internally in the master cylinder or the abs control block other wise you would have a small wet place on either a line or one of the callipers or wheel cylinder, or the back of the master cylinder, some of those can be hard to see but you would be replacing brake fluid often.
Air still in the line causes pumping of the pedal to achive the stop but not a sinking pedal after the stop.
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Old 01-08-12, 04:23 PM
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Old 01-08-12, 04:31 PM
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btw, how is MC fluid level? does it stay the same, or goes down? if it goes down, you have leak. if it stays same, you are either blessed with bad MCs in a row( I had three alternators, outta da box, in a row, bad, once. fortunately, it was 5 minute install), or you have air trapped somewhere, compressing under fluid pressure. the only area where it can happen is ABS module, as it has accumulators and actuators. now, for 93, I donno if you even have ABS, but if you do, some of them are very finicky.
 
  #8  
Old 01-08-12, 08:22 PM
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I am not losing a drop of brake fluid, & I know we got a good solid bleed of the system. I bench bled the MC prior to install & I bled a good while beyond any last evidence of air with crystal clean fluid from all 4 wheel cylinders. I had good pedal that night but next day it was falling again. I am convinced that I must be bleeding some brake pressure inside the ABS unit. This isn't good as new assemblies are $700+ so if I end up going that route I may hit a salvage yard. I plan to run some tests on my brake booster as well.

TexsFire
 
  #9  
Old 01-09-12, 04:39 AM
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Replaced 3 units in past 2 years
When you did this did you get air out of system and did it work right after you replaced the master cylinders?
If each time you replaced the masters it worked well then it must be the master or air because you bleed after the install.
I believe you said you get no air when you bleed it.
 
  #10  
Old 01-10-12, 05:39 PM
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well, for the year, this might work: re-bleed system once again, till you have good pedal. go out somewhere on a long stretch of rural road, bring her up to about 50, then slam on brakes as hard as you can, until she stops and repeat this maybe 10 times.
this is poor man ABS module bleeding. Otherwise, you destined to go see dealer and have them blead the system.
e.g., on my Camry, there are active actuators that operate ABS. the only way to bleed system is to have dealer scanner hooked up and use it to operate actuators inside ABS module. NO OTHER WAY TO DO IT.
 
  #11  
Old 01-11-12, 06:51 PM
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Hmmmm, worth a try I guess, nothing to lose except a little brake fluid!!

TexasFire
 
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