Presumably you never had this problem before and you let all the brake fluid run out when changing the caliper, air in the system?
Bleed it starting from the n/s rear, get somebody to pump the pedal half a doz times then hold it down firmly and tell you, you open the bleeder and watch the fluid run out, your assistant tells you when the pedal goes to the floor and you shut the bleeder. do it a few times , then the o/s rear then the n/s front and so on.
The caliper you got is second hand its the wrong type or its siezed.
You ****ed brake fluid all over the discs and pads
Its been laying for a while and it needs the pads burnt in with a good drive.
Lastly make sure all the calipers are working and the pads are ok and free in the calipers, means yur gona have to strip all the calipers take the pads out one at a time, hold a couple of levers against the pistons in em, get somebody to press the brake gently and allow the pistons to come out a bit then press them back in again.
Dont let the pistons come out too far use the levers to control it, and tell your assistant to go slowly and stop when you holler
Last thing part of your steering or suspension is fooked, but I'd go for brakes.