Paul, Realty, you need to check the operation of the rear braking system, as a first step. You have new callipers , so adjusted them up correctly first. Check you haven’t a broken handbrake cable :o :o ( very likely if you have that much travel, on the lever.) If you need new cable, change both, & g...
Straight forward, to do.
Just need a breaker bar & hub nut socket.
Make sure the car firmly held on axle stand.
Brake disc, pads, carrier & caliper have to come off.
Clean the ABS sensor, change the dust cover if it’s rotten