A friends XF ran out of brake fluid today.
He topped it up, but i don't think he has bled it properly (only did front wheels or something like that).
But now the symptoms he describes is that the pedal is very stiff and it requires a lot of force on the brake pedal to pull it up.
I always thought air in the lines would cause it to be spoungy.
Does this sound like air in the system or possibly the reason why it ran out of fluid in the first place.
I'm going to be looking after the vehicle for a week and i would like to fix this up while i have it. Any suggestions on what i should check first?

You are right air in the system usually gives a spongy pedal.
A hard pedal usually indicates a vacuum leak to the brake booster. However if it only occurred after losing the fuild I suggest dryed out/siezed caliper pistons, question is where did the fluid go? your friend needs to find the leak, brake fluid doesn't just disappear

how about telling him to bleed the system properly, and make sure the booster isnt f**k........

tell him to put his foot on the brake pedal, push down then start the car, if nothing happens, the booster is either f**k or has no vacuum

Agree with the booster and vacuum leak as the likely problem as this happened to my XF too.

drove it today. when you touch the brake, it makes a hissing noise inside the cabin. I'm guessing it's the booster has died. how much do ya reckon it would be to fix?
I'm thinking about buying it.. it;s in good nick, and he only wants $1000 for it. i think it's got about 190,000 k's on it

You could get it for less than $1000.
EA's are worth about $1000 and sometimes a little more for the same kilometres.

same issue as my EF - was the booster - cost me $50 for a second hand one from a wrecker... took about 20mins to replace