I don't always have a lot of luck rebuilting master cylinders, because rust can undermine a wider area than it appears.

However, you have not indicated you properly bled the brakes yet?
To do that, you first make sure the reservoir is full of brake fluid.
Then start at the furthest wheel, working closer as you go.
There are 2 stages to bleeding, where you first do it without pressure.
Open the bleeder.
Have someone push down once and hold it down.
Then put your finger over the bleeder before they lift up on the pedal.
With a little yelling back and forth, you can quickly bleed most of the air out this way.
Then you switch to pressure bleeding.
With the bleeder closed, you have someone pump up pressure.
Then you open quickly and reclose immediately.
Repeat until no more air at all, and pedal solid.

If the pedal is going to the floor there is either a leak in the system allowing air to enter or the brakes weren't properly bled after the master cylinder was replaced leaving air in the system. Either way you need to figure out why air is in the system and get it out.