If you have ABS, there is probably air trapped in the ABS module. Can be tricky to get it to shift.
Slacken each union in turn, starting from the master cylinder, get an assistant to push the pedal gently until fluid leaks out, without bubbles. Move along the pipe to the next union and repeat. Go all the way to the wheel. Repeat for all pipes.
As the ignition is turned on, the ABS module operates the pump for a few seconds. This can help bleed, sometimes. Open a bleed nipple, get the assistant to turn on the ignition. Close the nipple as, or just before the pump stops. Might need to repeat, lots.
Most master cylinders use cup shaped seals. Are you sure you got them all the right way around?
It is unusual to replace seals, as mostly the whole cylinder is changed. Stock seals can become hard in storage. Usual advice is to soak them in brake fluid for an hour before fitting. You could have seals that do not flex, so do not create the pressure.
A pressure bleed might be the answer. Pressure is applied to the top of the master cylinder, and forces fluid along the lines. Alternatively, a vacuum pump at the bleed nipple is anotehr way, but can be more difficult, as air gets drawn past the nipple threads, so looking like still has air in the system.
