# Did I destroy the master cylinder? (why are VW brakes so difficult to bleed?)



## motoracer47 (Jul 3, 2012)

To clarify... the brakes were working perfectly fine before I did this. 

So, this weekend, I installed the tyrolsport caliper bushings and stainless lines on the wife's CC. First issue I had was that the lines they sent did not screw all the way into the hard line fittings at each wheel like the stock lines did. I also noticed that the flare inside the fittings did not visually appear to be the same angle as the flare inside the original lines. Decided to give it a shot anyways. While trying to figure that out, the master ran dry. While I did not want that to happen, I figured it was not the end of the world since I have VCDS and can bleed the ABS pump. 

So, got all the lines installed, looked up the info, and what I found, says to do a normal bleed first, then bleed the ABS pump, then normal bleed again. Also, multiple resources said the bleed sequence was different than normal. LF, RF, LR, RR. Seemed really odd, but decided to follow directions. Also, I have a motive power bleeder, and that's how the brakes were bleed. The end result is this. I got the air out of the system, I can't get anything but clean fluid out of it at this point, but now, with the engine running, if you push the pedal, it initially holds pressure, then goes slowly to the floor. With the engine off, it holds firm. 

Where I am a bit confused is this. If it is a brake line fitting letting air in, should I see some bubbles come out when I bleed? If it was letting air in, should I see some fluid and or bubbles coming out of the fittings when pressure is applied? 

While doing research on this issue, I found where people have said that if you cannot get any air out of the system when you bleed, but the pedal slowly goes to the floor after initially holding pressure, that there is a good chance that the seal on the master cylinder piston has been destroyed, and its leaking pressure back into the reservoir. The explained reason is that on a car with some miles on it, that the bore inside the master wears over time, and that a lip can form. When the piston is pushed all the way in, the seal travels over the lip, cuts the seal, and the master is screwed. If that is the case, then what did it, was the ABS pump bleed procedure. I power bleed the system, so never pushed the brake pedal. However, when you do the ABS pump bleed with VCDS, the pedal will go to the floor, then the ABS pump will run and "pump" it back up.

Needless to say, I am going to put the original lines back on, and re-bleed to see if it is the lines. If after putting the original lines back on, bleeding out all air, and if the problem still persists, what then? Does that mean the master cylinder is destroyed? If so, then at least to me, that clearly means that bleeding the ABS pump destroys the master cylinder. 

Anybody else run into this? Thoughts, ideas?

BTW... just to make sure I have bleed the system as recommended online (LF,RF,LR,RR) and the traditional way (RR,LR,RF,LF) and no air either way.


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## motoracer47 (Jul 3, 2012)

Put he old lines back on, bled the system..... same thing. Pedal slowly goes to the floor. At a loss.... new master cylinder?


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## htr (Dec 22, 2014)

U need to be careful if u bleed using the pedal method.only push the pedal slowly to about 85% With the bleeder open,close bleeder release pedal.yyou can blow the seal in the master cylinder when bleeding if u push the pedal to far cuz it goes beyond normal with no pressure and inverts the seal,if that makes sense.


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## motoracer47 (Jul 3, 2012)

htr said:


> U need to be careful if u bleed using the pedal method.only push the pedal slowly to about 85% With the bleeder open,close bleeder release pedal.yyou can blow the seal in the master cylinder when bleeding if u push the pedal to far cuz it goes beyond normal with no pressure and inverts the seal,if that makes sense.



Yup, that's what I have been taught. I used a Motive power bleeder, so never actually pushed the pedal. However, when I did the ABS Pump bleed procedure with VCDS, the pedal went to the floor when the ABS pump activated. Unfortunately, I cant control that. 


I think the lesson here is this......Only do the ABS Pump bleed if you have replaced the master cylinder. If you let the master cylinder run dry, and have no choice but to do the ABS Pump bleed, you might as well buy a new master cylinder. If the car is new, or very low mile, you will probably be ok. If you have over 80K miles, letting the master run dry means replacing it. Kinda sucks they could not come up with another way to bleed the ABS Pump.


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