Changed wheel cylinders and now pedal only hits floor

Most commonly the oil would have been dripped off the unit while removing and air would have mixed or there would be leak in the system.

If the case is too much air, you could solve it be slightly loosening the hose attached to the caliper or removing the bleeder screw completely that comes from the reservoir. Once you see the oil coming out from the screw, you could tighten it and carry on the brake bleeding task.

Here are few other troubleshooting steps:

Reservoir : Check if the reservoir is full of oil. If you do remove the oil completly out of the system and you are refilling it again, please give some for the oil to fill the system

Issues with Bleeder Screw: Please check if there is no Blockage in the Bleeder Screw.

Hose: If the Bleeder screw is Good try removing the Hose that runs to the calipers. Check if the oil is comming out of it.

Caliper Unit: Check if the brake fluid is leaking from any other point. If the rings are loose on the caliper piston or if the caliper piston is damaged there are chances of oil leakage from the piston

On many brake systems you need to push the pedal down and close the bleeder valve before you let the brake pedal return,this is to avoid sucking air back into the system.

An other alternative is to use a brake bleeder with a return valve,this do only let brake fluid and air pass in one direction.

For the first solution you need to be two people one in the car pumping the brake and the other opening and closing the bleeder valve,you need to repeat this until all the air is gone so it only comes brake fluid when you push the pedal.

Be sure to keep an eye on the reservoir to avoid it running dry during this operation.