For those of you that upgraded their brakes to a 6 pot front and 4 pot rear setup or larger, did you need to upgrade the master cylinder? For those in the know, please chime in, as I'm having a problem with my setup. I've bled them several times but they are still "spongy" and do not grab as they should. What other problems can there be, besides air in the system? Thanks in advance for your help..