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#4674221 - 02/22/18 10:42 AM Weird Master Cylinder Behavior
artbuc Offline


Registered: 12/02/05
Posts: 1162
Loc: West Chester, PA
Re 2010 Acura MDX

This may be perfectly normal but I never experienced or noticed it before. Replaced brake fluid as usual using my Vacula bleeder. Started car and could not get a firm brake pedal. Just kept going to floor. Turned car off and pedal was firm. Turned car back on and braking was normal. Can anyone explain this? At first I thought I let air get into the MC but that just could not have happened because fluid level was always well above ram. Thanks for your thoughts.


Edited by artbuc (02/22/18 10:42 AM)

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#4674235 - 02/22/18 10:55 AM Re: Weird Master Cylinder Behavior [Re: artbuc]
Wolf359 Offline


Registered: 04/27/12
Posts: 4757
Loc: MA
That's how master cylinders fail. The seals inside the master cylinder went. No fluid loss but the brake pedal go to the floor. Time for a new master cylinder.

Or you still have air somewhere.

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#4674267 - 02/22/18 11:14 AM Re: Weird Master Cylinder Behavior [Re: Wolf359]
artbuc Offline


Registered: 12/02/05
Posts: 1162
Loc: West Chester, PA
Originally Posted By: Wolf359
That's how master cylinders fail. The seals inside the master cylinder went. No fluid loss but the brake pedal go to the floor. Time for a new master cylinder.

Or you still have air somewhere.


Thx for the quick response. I hope there is another explanation. The car is only 9 years old with 61k miles. MC was working perfectly before I replaced fluid. Also, I was not bleeding air, just replacing fluid so I canít see how there can be any air in the system. Iím wondering if it is normal ECU behavior resulting from the low level switch being activated?


Edited by artbuc (02/22/18 11:16 AM)

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#4674281 - 02/22/18 11:23 AM Re: Weird Master Cylinder Behavior [Re: artbuc]
KGMtech Offline


Registered: 10/17/14
Posts: 967
Loc: SW Ontario Canada
Potentially one of the ABS valves was leaking internally in brake hydraulic system, not enough to be a problem in regular use, but after the fluid replacement that portion of ABS system needed to fill up?
_________________________
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#4674319 - 02/22/18 11:42 AM Re: Weird Master Cylinder Behavior [Re: KGMtech]
artbuc Offline


Registered: 12/02/05
Posts: 1162
Loc: West Chester, PA
Originally Posted By: KGMtech
Potentially one of the ABS valves was leaking internally in brake hydraulic system, not enough to be a problem in regular use, but after the fluid replacement that portion of ABS system needed to fill up?


That is what I am thinking now. I applied sealant around the bleeder valve, something I have never done before. Usually I get a lot of air sucking past the bleeder threads such that the fluid looks like foam. With the sealant I got very few air bubbles meaning I put much much more vacuum on the system. I donít have a hydraulic circuit diagram of the ABS but I bet I sucked one of the valves out of position.

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#4674405 - 02/22/18 12:50 PM Re: Weird Master Cylinder Behavior [Re: artbuc]
The Critic Offline


Registered: 08/30/04
Posts: 19806
Loc: CA
You have air in the system, this is a common issue with vacuum bleeders. This is why I stopped using vacuum bleeders and use pressure or manual

Bleed the system manually in the correct sequence (should be lf, rf, rr, lr) and it should resolve your problem.
_________________________
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#4674473 - 02/22/18 01:36 PM Re: Weird Master Cylinder Behavior [Re: artbuc]
littleant Offline


Registered: 10/10/17
Posts: 388
Loc: Vermont
The critic is correct. Pressure or manual bleed. I believe you got a slug of air when bleeding.


Edited by littleant (02/22/18 01:40 PM)
_________________________
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#4674481 - 02/22/18 01:39 PM Re: Weird Master Cylinder Behavior [Re: The Critic]
Trav Offline


Registered: 11/20/06
Posts: 18893
Loc: MA, Mittelfranken.de
Originally Posted By: The Critic
You have air in the system, this is a common issue with vacuum bleeders. This is why I stopped using vacuum bleeders and use pressure or manual

Bleed the system manually in the correct sequence (should be lf, rf, rr, lr) and it should resolve your problem.


^ This
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#4674484 - 02/22/18 01:40 PM Re: Weird Master Cylinder Behavior [Re: The Critic]
artbuc Offline


Registered: 12/02/05
Posts: 1162
Loc: West Chester, PA
Originally Posted By: The Critic
You have air in the system, this is a common issue with vacuum bleeders. This is why I stopped using vacuum bleeders and use pressure or manual

Bleed the system manually in the correct sequence (should be lf, rf, rr, lr) and it should resolve your problem.


You have the right sequence. Have used my Vacula probably 10-15 times to replace and/or bleed brakes. Never had a problem. Just finished a long test drive and brakes worked perfectly...pedal feels great. How could that be if air is in system?

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#4674494 - 02/22/18 01:44 PM Re: Weird Master Cylinder Behavior [Re: artbuc]
littleant Offline


Registered: 10/10/17
Posts: 388
Loc: Vermont
I think the air may have bleed back and out through the master cylinder.


Edited by littleant (02/22/18 01:44 PM)
_________________________
I can't prevent getting old but I REFUSE to grow up.
2010 Mountaineer 4.0
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#4674513 - 02/22/18 02:02 PM Re: Weird Master Cylinder Behavior [Re: The Critic]
maxdustington Offline


Registered: 01/21/17
Posts: 678
Loc: Toronna
Originally Posted By: The Critic
You have air in the system, this is a common issue with vacuum bleeders. This is why I stopped using vacuum bleeders and use pressure or manual

Bleed the system manually in the correct sequence (should be lf, rf, rr, lr) and it should resolve your problem.


How does this happen? I've seen the argument made that air gets pulled in to the system somehow but I have never had anyone with first hand experience describe it in any detail.

I use an oil extractor to bleed my brakes and have never had an real issues but I always seem to get bubbles constantly, especially on the RR caliper.
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#4674521 - 02/22/18 02:09 PM Re: Weird Master Cylinder Behavior [Re: maxdustington]
artbuc Offline


Registered: 12/02/05
Posts: 1162
Loc: West Chester, PA
Originally Posted By: maxdustington
Originally Posted By: The Critic
You have air in the system, this is a common issue with vacuum bleeders. This is why I stopped using vacuum bleeders and use pressure or manual

Bleed the system manually in the correct sequence (should be lf, rf, rr, lr) and it should resolve your problem.


How does this happen? I've seen the argument made that air gets pulled in to the system somehow but I have never had anyone with first hand experience describe it in any detail.

I use an oil extractor to bleed my brakes and have never had an real issues but I always seem to get bubbles constantly, especially on the RR caliper.


Right, bubbles come from air being sucked past the threads but it is immediately entrained in the brake fluid and is extracted. The only time I got in trouble was many years ago when I used my Vacula to suck old fluid out of the MC and I went too far.

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#4675097 - 02/23/18 06:56 AM Re: Weird Master Cylinder Behavior [Re: artbuc]
artbuc Offline


Registered: 12/02/05
Posts: 1162
Loc: West Chester, PA
Originally Posted By: artbuc
I donít have a hydraulic circuit diagram of the ABS but I bet I sucked one of the valves out of position.


Duh, of course I have the hydraulic circuit diagram...it is in the FSM. It looks possible, maybe likely, that I emptied the accumulators. In any event, FSM says no special bleeding procedure required for ABS unit. For kicks and giggles I did manually bleed front brakes (first two in FSM sequence) and got zero air. Did another test drive with repeated hard braking and performance was excellent. At this point I have to conclude all is well. Thanks for your help!

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