I've read in several places that the master cylinder on an ABS-vehicle CANNOT be bled of air without a scan tool.

Only a little air in the lines can, however...

 

The master can always be bled, it's the 4 wheel ABS valve that has to be bled on some applications with a Tech 2 scan tool, not all though.

 

I am having this same problem on a 91 Vette that I recently aquired. I have bleed the sysem 4 time with a vacume, I have to believe there is no air in the system. I have replaced the master cylinder, and check everywhere for leaks. How does the ABS valve cause this? Is there a way to eliminate the valve to see if the problem goes away? Mech259 suggests pinching off the rubber brake hoses will that show me it the ASB valve? Sadly I don't know what else it could be.