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9n3 1.4tdi nsf brake binding but only when engine is running.


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Walterbrown

Hi guys, it's been a while since I've owned a polo (white 6n 1.4cl). Bought my girlfriend a 9n3 1.4tdi sport about a year ago, did all the general maintenance and has been problem free. Until a couple of days ago she was coming off the M1 and the brakes went completely to the floor, luckily we live 2mins from the M1 so she managed to limp it home. I finished work and had a look, looked for any fluid leaks which there wasn't. Checked the fluid level which was fine and pumped the non existent brakes for a minute and had no drop in fluid level. So diagnosed as a failed master cylinder. Changed the master cylinder yesterday, flushed and bled the system. Yay! We have good solid brakes. Went on a 5min drive and they started to bind. Jacked car back up and the nsf brake is binding. Stripped the brake down and cleaned and greased them. Pads nice and free. Piston pumped out and pushed back in nice and free. Rebuilt and pumped back up, Wahoo no binding. This was short lived, as soon as you pump the brake with the engine running it binds and won't release. So with the caliper being nice, smooth and free, starting to clutch at straws I bought a flexi hose thinking it may have broken down internally. And that with the added pressure of the servo it was causing the issue. But this made no difference what so ever. I'm thinking it some with either the master cylinder being faulty, or something with the vacuum pump/servo.

Any help is much appreciated.

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  • Walterbrown

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Walterbrown

Just got the car jacked up and wheels spinning freely. Started car and pumped brakes. Nsf binded and solid. Turned engine off and still binding, removed vacuum line from pump to relieve pressure and still binding.

 

Update, got it to bind again after removing the vacuum pump end which didn't change anything, then realised there is a one way valve fitted so removed the servo end and it instantly freed up. My question is why is it only binding one brake?

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nige8021

First Welcome to the forum.

 

When you bled the brakes, did you put the ABS unit into service mode with VCDS ? as without doing this you could have trapped air in the pump unit

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The one way valve on the vac hose to the servo is a pretty common fault. You can test it by blowing through it.

 

I'd guess there's a bit of uneven wear or a bit of corrosion stopping the brakes releasing evenly. 

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Walterbrown

Hi thanks for the reply, when the engine is not running the brakes never stick. Fire the car up and pump the pedal 10/15 times and it locks solid. When the car is running the servo feels like it's operating correctly. Which gave me the impression that the vac pump and valve were working. I'm struggling to blow down the pipe at the minute, but the first fews times it seemed fine.

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How are the brakes with the pipe off the servo?

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  • 2 weeks later...
caretakerplus

Did you manage to sort this problem?

 

I was wondering if, when you pushed the caliper piston back into its cylinder, you released the bleed nipple and allowed the displaced fluid out of the system?

Failure to do so COULD cause dirty fluid to be pushed into the ABS unit.

 

Regards

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