Brake Issues

[454SSguy]

So I'm having some brake issues. I have 2 master cylinders (stock one and a brand new Willwood unit) and they both have same symptoms. Not alot of pedal and when we crank it to put some vaccum in the booster it gets really soft and is very very slow to come back up. All rubber lines have been replaced with braided stainless and we have triple checked everywhere for leaks so I'm thinkin it's either the abs unit (probably gonna just tear it off anyway), or the proportioning valve. Any ideas...

 

[Joesgarage]

Does it change anything when you disconect the wires from the ABS?

 

[towman]

crazy as it sounds you need to push fluid from the calipers back into the master cyl, air gets trapped in the abs and is a real pain to get out, you will see air bubbles going into the master and probably puffs of dark fluid