98 Disco i changed the rear calipers and now i have a fault on the ABS the pedal goes hard or i get a very noisy rattle from the front can i fix it with out a computor

 

You have a bad/messed up wheel speed sensor.
The noise you here is the ABS modulator pumping the brakes and the hard pedal is the ABS modulator working against you.
Since you just worked on the rear brakes check that wheel speed sensor and make sure that it is still seated properly.