Weird Master Cylinder Behavior

[artbuc]

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.

 

[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.

 

[artbuc]

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 can’t see how there can be any air in the system. I’m wondering if it is normal ECU behavior resulting from the low level switch being activated?

 

[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?

 

[artbuc]

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 don’t have a hydraulic circuit diagram of the ABS but I bet I sucked one of the valves out of position.

 

[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.

 

[littleant]

The critic is correct. Pressure or manual bleed. I believe you got a slug of air when bleeding.

 

[Trav]

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

 

[artbuc]

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?

 

[littleant]

I think the air may have bleed back and out through the master cylinder.

 

[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.

 

[artbuc]

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.

 

[artbuc]

Originally Posted By: artbuc

I don’t 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!