Hi again! Only time I have seen this problem is a oil based or contaminated fluid had been put in the resivoir, if it was it will cause exactly what you are saying, and it won't let the fluid be bled out because it swells all the hoses up, If this is the case it will be very expencive to repair, You can pull a rear wheel cylinder and see if the rubber parts are swelled up to know for sure, other wise You need to start cracking lines open to see where the blockage is front or back, The ABS system has to blead with a scan tool.
ASE Certified Technician
24yr gm Drivability & Electrical
Hello! Have You found anything on the brakes?