Starting at the caliper, work backwards up the brake line and loosen off each joint in the line, ie hose to caliper, steel pipe to hose, ABS pump to steel pipe. If you get to the ABS pump and still no fluid, that could be your problem. Just to make sure you need to do the same thing but working forwards starting from the master cylinder, only this time there should be fluid seeping from the joints as you undo them.
This way you should be able to pinpoint the problem without changing bits willy nilly.
It will obviously help if you can find out which pipe/s from the master cylinder goes to the front O/S, although its not critical it just saves you undoing joints that you don`t need to.
HTH