I had an issue with the master cylinder where the little black rubber caps (for brake level sensor?) were cracked and the master cylinder would not build any pressure. Air (and brake fluid) would just leak. What I did was apply a TON of gorilla glue to the cracked cap and then put a hose clamp (3/4 inch?) around and tighten it as much as possible. I haven't bled the brakes yet but the pressure was holding with minimal air leaking (still heard a hissing sound, just was tolerable).