I have a 2009 2.0 TSI and replaced the seals on my vaccum pump today because there was a bad oil leak. The job was pretty straightforward and the seal kit included 3 o-rings and the gasket between the pump and the head. Everything seemed to be fine until I pulled up to park and after pushing the brake pedal a couple of times it went hard and the brakes almost didn't work. If I drive off it's fine again after the revs get up a bit but the same thing happens if I pump the pedal at low speed.

I've tried pumping the pedal with the engine off, then hold and start the car and the pedal doesn't go down. I think this suggests a faulty booster but it's a bit of a coincidence! I've checked all the vacuum hoses are secure, the only one I had off is the one at the back of the vacuum pump. I can't see anything out of place and if I get someone to pump the pedal I can't hear any leaks under the bonnet.

Does anyone have any idea what I might have done wrong? I don't think the vacuum pump can go on the wrong way, there's a channel on the camshaft it has to line up to.

When I opened the pump itself to replace the main o-ring there was a small part that dropped off the rotor inside, it wasn't attached with anything, it just slid on the end. I put it back and checked it was the same as the other side. I suspect if this was wrong the pump wouldn't work at all though.