300tdi rear brakes

Hi, wondered if anyone could help..when I press the brake pedal the front brakes lock up and the rear brakes don't seem to do much.

Im out in the alps, so when it was skidding to a stop on some snow, I popped my head out of the window, the front was locked, the rear wheels were still turning.

Im guessing this shouldn't happen? and it has disks all round.

I wouldn't really know where to start to try and solve it...and im in the middle of changing the p gasket and water pump...no heating at -15 is not fun!

Thanks

check the fluid level first , seem to remember theres a shuttle valve on the brake line as it goes down the bulkhead from master cylinder , known these to stick in cold weather . good luck .