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 ***

[Crasher]

I think your servo is faulty due to water ingress.

[wonkey]

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

[Crasher]

Keep those plenum chamber drains clear!

[wonkey]

With the servo and battery out I took the chance to remove the rubber grommets and clear the whole area. Don't have a problem currently, but will seal the pollen filter box when I get some sealing tape.

[Crasher]

Leaving the drain valves out is not a good idea, it allows fumes to be sucked into the heater.