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Discussion Starter #1
Hi:

I have a 2005 Passat TDI which I recently purchased and drove from Quebec to Ontario. I also drove it for another 2 weeks prior to performing some major work on the vehicle. So far I changed the Timing Belt, Camshaft and Drive Axels ... I even painted the drivers side front fender.

Anyway, my latest concern is that after starting the car for the first time, the brake pedal comes up quite firm, but slowly sinks ... right to the bottom!

Funny thing is that I did nothing at all to the brakes.

Even when changing the CV shafts, the calipers were simply hung on the side. Installing back over the rotor was the expected snug fit as they were not moved at all during CV shaft replacement.

Well, I've searched the net and some say this is normal operation for my car (HUH ?) and others say the master cylinder needs replacing (sigh - I'm getting sick of working on this car!).

I don't understand ... I did not notice this condition prior to my repair activities, none of which had anything to do with brakes.

One final point, the pedal does not sink when the engine is off ... no matter how hard I push. However, when the engine is running - SSSSSSSsssssink..... %&#!&*$ !
 

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Check the back of each wheel, you might have a small leak or you have air in the system. I've never heard of this being normal, I think you've read dodgy information. If it was the master cylinder I would expect the pedal to fail now and again as the seals deteriorate.:)
 

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With the car running:
If you step enough so that the car doesn't move, that is normal. If you stand on the pedal hard and the pedal slowly sinks to the floor, that is also not normal.

With the car off: your described pedal behavior is normal.

I've also got this problem and have given up trying to fix it since the brakes and ABS all seem to otherwise work well. The brake system has been bled multiple times with the ABS pump, both with a pressure bleeder and with manual pumping. My best guess is that there's something about the master cylinder and ABS line or pump which causes some Passat to have this problem. Maybe there is air somehow trapped somewhere in the cylinder or lines but I've bled the ABS pump a few times.

A new master cylinder and bench bleed, where you fill the cylinder with fluid and pre-bleed it before installation might do it. However, it spills fluid everywhere (the fluid will quickly eat paint so you have to immediately wipe up and rinse off spills) and considering that the master cylinder on Passat is annoying to remove (refer to the writeup), I don't feel like doing it. If there's a braking problem I'll do it, otherwise I'm content to leave it be.

My car has ESP. The same MC is used for both ESP and ASR only cars but the ABS pump and lines are different. I don't know if that makes any difference.

Also, post moved to mk4 section.
 

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Discussion Starter #4
I'm planning on disconnecting the vacuum to the brake booster and see if the pedal sink still occurs (with engine running of course) ... I'll push real hard using both feet. If pedal sink still occurs, then I know another system must be responsible, namely the ABS. I'll then disable the ABS and see if that stops the pedal sink issue.
This is a real mystery and the fact that so many Passat owners have to live with it ... with no known fix is a real inconvenience. I mean, something must be happening ... and the only thing I disturbed with regard to the braking system were the ABS wheel sensors by removing and reinstalling after the new axels were put in. Could this really be the cause ??? Somehow I doubt it as they are only proximity sensors which send a frequency pulse back to the control unit to indicate wheel rotation speed. Like I said, before all this work, my brake pedal was fine and did not exhibit any pedal sink ... how do I know you ask? Because when I purchased the car, I drove it back all the way from Quebec to Ontario and a couple of times there was the issue of stop & go traffic ... I'm sure I would have noticed. I also drove it a few weeks before commencing repairs.

Anyway, if pedal sink still occurs after brake boost disconnection, then another system must be responsible. Why brak boost disconnect ... because this allows a pure comparison between brake force with the engine on vs. the engine off. If the pedal still sinks, then something else other than the master cylinder is responsible ... I still suspect something with the ABS.

Regards, Troy
 

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Its a master cylinder problem the seals have deteriorated and need replacing if you can get them or a new master cylinder. This happened to me with my last car. ;)
 

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Discussion Starter #7
Hi Chitty:
I was able to remove the vacuum line from the Brake Booster and I performed the definitive test.
With the engine off, the pedal comes up solid and I can't get it to the floor even with both feet and a hernia.
With the brake booster disconnected from vacuum and the engine on ... the pedal behaves as if the engine is off.
When I re attach the brake booster vacuum ... the pedal once again sinks to the floor
I think this simple test totally eliminates the ABS system, air in the lines, etc.
I believe it is a problem with the Master Cylinder and the brake booster is able to multiply the applied force greatly.
Correct me if I'm wrong, but from my understanding of brake boosters, there is nothing "soft or compliant" between the pedal rod wich goes in one end and the output rod which makes contact with the Master Cylinder primary piston ... in other words, between the pedal and the master cyclinder, there is nothing but rigid metal parts ... the brake booster simply multiplies the force applied.
Regards, Troy
 

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Troy it still sounds like master cylinder seals. On a direct acting servo it acts directly on the main piston as you say. With the servo acting the fluid can be drawn into the vacuum pump ("exhauster"). ;)
 

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Discussion Starter #10
Hi Keith:

I decided to finally take her down off the jack stands and have a careful test run.
Everything works great except for the slow pedal sink (excessive force required)

I find that with the incredible brake boost this car has, I hardly have to touch the pedal which is probably why I did not notice the issue before I started all my repairs ... Timing belt, Camshaft, Axels, Tranny fluid, brake bleed, Fender rust repair & paint ... more.

I also performed a hard stop on a back road and she stopped with the ABS doing its thing and all, so I'm confident she's safe.

It's getting cold here in Ontario Canada, so I think I'll call time on this project for the time being.
I'll have to commence in the spring with the Master Cylinder replacement.

Question: Do you know if I can I remove the Master Cylinder without removing the brake booster?

Thanks for all you advise.

Troy
 

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My car has ESP. The same MC is used for both ESP and ASR only cars but the ABS pump and lines are different. I don't know if that makes any difference.

Hey Chitty, I think your mistaken its not your car but the computer
 
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