The girlfriends having issues with the brakes on her Fiesta 1.2 2010. 

99% of the time the brakes work fine, but every now and again the pedal sinks straight to the floor, the abs turns on, but no braking force is applied. This seems to happen at time when traction is lowered, patches of gravel etc, however this is more than just the abs kicking in, you can feel the pedal go slack and you have to pump it to get braking back.

My first thought point at the master cylinder, but its an expensive bit to replace on a hunch. Has anyone else experienced this, or has any diea what the cause may be?

Thanks in advance.

Could be an abs pump solenoid or control module failure.
When was the brake fluid last changed?
Contaminated fluid (water, dirt etc) can cause intermittent brake effectiveness.
I'd get it checked out immediately, the next time it fails may be in an emergency.

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could try a good fault code reader on it to see if any fault codes logged, but I would have thought ABS light would come on if it was actually detecting the fault. Sounds pretty dangerous to use at present. Brakes are dual circuit so in theory you should only lose brakes on 2 of the wheels - in theory. hard to sort out if you can't recreate the fault to order.  have you checked that fluid level definitely not gone down and no sign of damp around brakes on all four wheels from a leak (although i would not expect a leak to be intermittent)