K1500 Master Cylinder question
 

Hey, I have been trying to bleed the brakes on this truck. First night my buddy smacked the master cylinder lid on and popped it off the aluminum part; I replaced the entire cylinder/aluminum part and it came assembled with new rubber seals and a new lid & gasket.
I bench bled the master cylinder and gravity bled the brakes. The peddle is still soft (I will manual bleed the brakes with another person tomorrow).
I just noticed when I push down on the lid, I hear a very slight hissing sound. It looks like it's coming out the rubber bushing underneath the part of the master dedicated for the rear brakes. Do I need to replace those bushings again?! They are literally brand new along with the entire master cylinder unit. Will they (if they did unseat) reseat?

Try spraying windex around it and see if air bubbles form when you push down.

If it does, is it defective?
I understand the pressure has to go somewhere and the rubber seals would be the easiest spot for it to release pressure but it is brand new. If it's leaking will it reseal or should I be concerned?