Brakes not bleeding right

[Suprareal]

So ive got the BM 50 Master cylinder(z32) ( i bubbled the end of the line for the middle input) and just started putting all my lines back on to start bleeding them today.
Everything was going good at first. started with the back right caliper and my brake pedal felt really good better than it ever has. But when we started bleeding the front right (the line that is bubble flared) the pedal got really hard to press and the little black cap shown in this picture in the middle popped off and fluid sprayed out of it.


IF i try bleeding the pedal is super stiff and feels like ill break something if i push it harder in and i can hear the cylinder sucking fluid in. And it sprays fluid out the hole every single time the brakes are pressed.
What exactly is this hole with the black cap?
And does anyone know if this is happening because of the bubble flare on the line because i dident put in the fitting? I dont know what to do from here.