Vito brake pedal spongey, but only with engine running?!

[TroubledSoul]

My W639 Vito has a really spongey brake pedal. I have bled the brakes, twice now. Once with the engine off then again with it running to make sure the ABS pump is primed. If I pump the pedal with the engine off, it stays rock solid. If I do it with the engine ON, it slowly sinks right down. I'm a bit puzzled and have an MOT coming up on Saturday morning so need to figure this out ASAP.

Am I right in thinking that it can't be the master cylinder since the pedal won't sink with the engine off?? I also tried greasing the sliders on the front calipers and I don't believe either front caliper is seized, having had a look at them last night.

Any ideas anyone? I do have a brand new MC I can throw on but I'd rather return it for a refund if it's not the problem. If it is, then I'll swap it tonight.

[TroubledSoul]

The Wookie said:

Probably master cylinder on the way out. You're generating more line pressure with the engine running and the servo working for a given pedal input and it's likely blowing past the seals in the M/C.

If you really stand on the pedal with the servo depleted you will probably find that it still creeps.

The only alternative is that something odd is going on with the servo

I wondered about the "generating more pressure" thing. Sod it, I'll bite the bullet and change it.

[TroubledSoul]

Hmm... I don't really want to be disconnecting anything though. I'd rather fix it properly. I guess as a test I can try that but it's not something I'd want to be using as a bodge.

[TroubledSoul]

I've spoken to other owners and they've said that it's quite normal on these. Testing last night revealed that, despite the crappy feel, I can lock the brakes and kick the ABS in so they must be working OK... Just not very confidence inspiring.