Brake issues after caliper change

You'll defo have air stuck in it, the rear calipers can be a total pig to bleed.

If the pedal creeps to the floor with you applying constant pressure then you've probably got a leak. That or the seals in the master cylinder have gone, but thats really quite rare.

I had a problem like this that drove me mad for weeks.

Brakes started off perfect then i did a track session at audi driver which went fine. Got in the car to go home at the end of the day and pedal was on the floor, three pumps required to stop sorta thing.

Got home and assumed i'd just boiled the fluid, so i bled the system thru to sort it, at which point the pedal sorta started working but was long and mushy. Two master cylinders, new brake pads, new rear calipers and various other bits later and the brakes still werent right. The only fault i found was a weeping rear caliper, which is what i assume caused the initial issues. It got to a point where it worked well enough and would safely stop the car, just felt really nasty, so i gave up and ignored it for months. Eventually, months later, i bled it all one last time with a vacuum bleeder and it more or less sorted it.

If its safe to drive, then i'd go do some miles in it, and then re-bleed the rear calipers. I think fluid gets stuck in the handbrake mechanism and thats what causes the problems. The vibrations from driving around shakes the bubbles free and then they bleed out properly.