Sinking brake pedal after fluid replacement

[thedad]

I have replace the brake fluid on my car using a pressure bleed system.

But, now, when the car is started and I press on the pedal, it sinks!

Prior to this it was really stiff right at the top, but I think some of that was due to the front calipers where one had seized and the other one was close to the same so not much movement before they bite.

What have I done wrong