I have a 2004 2L CRDi AWD Tucson. When the car is not running, the brake pedal is solid and remains so under pressure. Once I start the car, the pedal sinks a little under pressure but never all the way down. The brakes seem OK if a little less sharp than they used to be but will stop the car no problem. The pedal initially sinks when applying pressure and starting the engine as it should under servo assist, but will continue to drop slightly with constant pressure.

I can not find any obvious leaks and my suspicion is that the seals on the master cylinder may be letting oil past internally. Can anyone give any further information please. I ordered a new cylinder online but although it said it was a compatible one, when it arrived it had larger threads for the pipes and also a third connection which mine does not have.

The first image is the correct type and the second is the new one I received:






Thanks.