But like OP. Bleeding alone doesn't fix. Or in my case. Doesn't fix for long. It progressively gets worse. Something is certainly sucking? (Sorry for the pun)
Right. If the seals in the master cylinder are bad or not seated correctly the fluid can just be exchanged between the ports or chambers. Pressure can be lost as it starts to build if it goes past the seals, pedal stays soft even though no air comes out of the system. You can’t build pressure in the lines if the seals around the piston are leaking badly enough.
In the OP’s case he didn’t bench bleed the master cylinder, so there may be air trapped in the master cylinder keeping it from building pressure. Or it could be faulty seals in the master cylinder. Assuming that the rest of the system was bled properly as described and there’s not air trapped anywhere else. But you and the OP may have different issues.
