Soft Pedal With Engine Running?

[LikesBikes]

Second time I posted this - For some reason the title disappeared when I posted it just now on my phone? If a mod is reading could you delete the other one please?

Anyway, just changed front discs and pads on The wifes runaround - a bog basic Vauxhall Corsa. The pedal is nice and firm until the engine is started then it sinks to the floor and no amount of pumping will sort it. Switch off and it goes firm again.

Is this a knackered servo or master cylinder or could it be something simpler (and cheaper)?

Any help or suggestions greatly appreciated?

Cheers gang, Darren.

[pk500]

Its quite common to roll the rubbers in the master cylinder when you pump pedal back up normaly do one side then build up pedal before doing other side, easy check is start car and put your foot on the brake and see if any bubble come up in the resovior bottle