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Soft brake pedal after full flush/bleed

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#1
tynor

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Please help me brainstorm.   I replaced brake calipers on the Miata.  Completely flushed the fluid but I'm still getting what I think is a too-soft pedal.

In the course of bleeding, I pressed really really hard on the brake and heard a little "pop" from under the hood.  Master cylinder?  Booster? No leaks and no amount of further bleeding is showing any air.  Yet, the pedal still doesn't feel as hard as I remember before I started.

I'm tempted to swap in new master cylinder.  And maybe booster.  But that just seems "wrong" to throw parts at the problem without a theory of what might actually be wrong.

Ideas? 

Steve

 


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#2
JDaoust

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This has happened to me before.  I'm 99% confident that you bent the backing plate of one of your brake pads.  I had this happen twice last year.  The first was when I was putting on new calipers, and when pumping the brakes I heard a 'pop'.  What happens is when your caliper was backed off for installing new pads when you put everything together one of the pads slid off the rail (even slightly).  I had the same thing happen when replacing the pads later in the year.  That plate ends up acting like a spring and keeps pushing back your piston.  I hope this helps. 


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#3
tynor

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Ding! Ding! Ding!  We have a winner.

That's exactly what happened.  One of the drivers side right front pads was in fact bent.  I replaced the pads and I'm back to a nice hard pedal. Thanks for the pointer - I don't think I'd have thought to check that and would have suffered with a soft pedal until it was time to swap pads.

 

bent pad backing plate

 


Steve Tynor

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Atlanta, GA

 

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#4
JDaoust

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Awesome.  Glad it was an easy fix. 



#5
Tom Hampton

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Ding! Ding! Ding!  We have a winner.

That's exactly what happened.  One of the drivers side right front pads was in fact bent.  I replaced the pads and I'm back to a nice hard pedal. Thanks for the pointer - I don't think I'd have thought to check that and would have suffered with a soft pedal until it was time to swap pads.

 

 

I'm pretty sure we've all done that, once (or three times).  You learn to check those pads after a switch as you reseat the caliper pistons.  check, pump, pump, check, pump, pump, check.  repeat until seated.  Then check one more time before pushing on the pedal hard. 

 

The soft pedal actually goes away after the backing plate fully deforms (typically after its been heated).  The first time I did it, I didn't notice until I got back from a track day and removed the pads to inspect them.  The pedal was a little soft during the first session, but cleared up after a couple of laps.  I didn't think anything of it, at the time.  I got home and the brakes were making a horrible noise, so I took them off to find the bent backing plate. 

 

I think I did this about 3 times before I finally learned.  I still have an odd number of spare front pads as a result. 


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