On a 94 Grand Prix with 4 wheel disc brakes and ABS, I have no brake fluid coming out of the ABS unit to the two lines that run to the rear calipers. I checked the fuse and it is OK, and there is fluid pressure from the ABS unit to the two lines that run to the front calipers. I suppose pressure to the front is a default status. What prevents pressure from going to the rear? Thanks in advance for any advice.