Loss of braking after 3-4s

[wonkey]

Car: Volkswagen Passat 1.9 TDI

Symptoms:
Sharp braking works as expected, the car comes to a halt quickly. Slow braking (imagine slowing down for a junction) works for 3-4s and then seems to stop. Pushing extra hard on the brake pedal doesn't help to stop the car. It feels like the brake pedal is at full travel, but the brakes aren't slowing down the car.

Tests:
If the car is started, allowed to run for a minute to build up the servo vacuum, then turned off, it is possible to hear a faint air hiss from around the servo area. It's not loud enough to pin point exactly where though.

The vacuum hose has a one way valve in it, just before it enters the servo. I've taken this hose off and tested it in water, the valve works as expected.

I've disconnected part of the vacuum hose from the pump and run the engine. You can feel suction, not enough to pull your finger into the pipe, but enough to give some resistance to you pulling your finger out of the pipe. I've no experience of what the level of suction should be here, so have assumed it is ok.

This led me to believe it is a problem with the servo, or one of the seals on the servo. The air escaping noise isn't made any louder by wiggling the vacuum pump hose where it enters the servo, so I assume that seal is ok. The other seals are between the servo/cylinder and between the servo/car interior.

After a lot of struggling I managed to separate the brake cylinder from the servo (space is very tight!). The seal looks in good condition, although the cyclinder itself is discoloured (see picture).

I'm hoping for some more opinions/tests or just clarification I'm heading in the right direction! I was going to replace the seal on the cyclinder, put it back together and test again. Now I've seen the condition of the original seal I'm not convinced it will fix the problem. I'd like to see the seal at the rear of the servo as well before putting it all back together, although I'm not looking forward to releasing the servo, it doesn't look like a pleasant job, plus I need a special tool!

*** Pictures coming soon ***

[martin1810]

Because these servos suffer from water getting in and damaging them internally, I would replace the servo. Your problem certainly sounds like a faulty servo, as in loss of servo assistance. Have you checked the piping to the small vacuum reservoir?

[Crasher]

It does sound like servo water damage.

[cazyp]

Not that the excellent advice above needs a thirding - but I agree and would replace the servo.

[wonkey]

Old servo had signs of water ingress when removed, as did the end of the master cylinder. New servo, problem solved, thanks!

[Mr auto]

I am having the same problem with my 2013 Volkswagen CC, the brake pedal sinks to the floor when I engine is on, iv bledd the brake lines for air, iv got the servo tested, and have had new callipers put on, last thing is the master cylinder I think is the problem.? Any information would be appreciated thanks

[Crasher]

@Mr auto you will get a better result if you start your own new post in the CC forum, this post is 13 years old... Have the master cylinder and or ABS pump been run dry?