I have a 2001 F-150. I replaced all 4 brake calipers and was able to gravity bleed the fronts no problem. However I can not get any fluid to rear calipers even with a helper pumping the pedal while I open the bleeder screw.

 

Some vehicles had a height limiting valve that based on the load the vehicle carried , controlled the application of hydraulic pressure to the rear brakes to prevent rear wheel lock up. Not sure if that is on your vehicle.

 

Thank you. I will check into that.