im unsure of what model u have is it a75 e 15 u need to check the air pressure from the foot brake valve to ur master cylinders .there shood be test points here.check ur load valve isnt stuck (above rear axle)it shood move when foot brake is pressed.brake callipers tend to sieze up as do master cylinders and the rods on the air cylinders pushing the master cylinders .also check brake fluid level . hope this helps
nigel mc lean, September 2007