On my civic I replaced the master and slave cylinder and had bled it, and had pressure for about 7 pumps then the pedal began to fall to the floor and now when I push the pedal there’s like a hissing air sound coming from the master cylinder as it just sucks back to the ground

 

did you bench bleed before install?

 

no I did not
is that what the problem is?
and would I have to buy another master cylinder

 

Maybe.

 

pull it out and bleed it then put it back in, there is probably a bunch of air inside of it.

 

i had posted a video on YouTube if anyone wants a better look