fiesta brake problem

hi, i have a 1.4 petrol fiesta with durashift and and abs. there is a problem with the brakes, when the pedal is pressed the brakes start working but then the pedal goes soft and goes to the end. if i am stationary and press the pedal it goes firm and then after a while goes soft and goes to the floor again. i had the brake master cylinder changed a few months ago and it seem to sort out the problem, but now its the same again. any ideas if its the same again or a problem with the servo. all fluid levels are correct and theres no leaks.

thanks

Firstly I'd stop driving the car till it's sorted. I does sound like the master cylinder again, was it a new one fitted and is there any warrenty on the work carried out by the garage ?

used a second hand one, which was in 'good condition' .a friend put it in for me, so thats not a problem. just was curious if it was the same problem or maybe it was the brake servo.
thanks for the reply.

If the servo had failed you would have to use the bake pedal a lot hard without them working as well

ok thanks will get checked by a garage first nd see what they think, but looks like the same problem.

Personally I would never use any "second hand" brake components, unless they have been refurbished by the manufacturer.