Break Problems :'(

I recently finished the mz6 swap for the rears, but now i have problems. I can build up pressure on the pedal while the car is off, but as soon as i turn her on I lose everything. I've completely flushed the system to no avail. from what i've read, it could be the master cylinder, but my brakes were working fine before the switch (except a seized rear right caliper), as well as i kept the fluid from getting to low. On top of all this, my rear right wheel won't spin at all.
I don't want to keep going through brake fluid through her, and I'm at a loss for what my next step should be

It's not uncommon for a master cylinder to fail after bleeding the brakes. Pumping them causes the pistons to travel to an area in the bore that they have not been in for years, which can have corrosion or debris in it, and the result is damaged seals. Sounds like there is still air in the system to me. You will probably need to bleed each wheel.

As for the rear caliper that won't release, open the bleeder and see if it lets go.