98 Blaser 4 wd with ABS anti lock brakes. Son had changed the rear calipers. I can not bleed the rear brakes as it seems the combination valve has locked out the rear system and I can not get it to slide back over. I have tried bleeding the front in hopes that it will slide back but all I can get to bleed is the front. Seems this system is more complex than a regular system.Anyone know how to get the combo valve back to the proper position so brake fluid will travel through the rear lines? Thanks