You might just have to bleed all the air out of the system and lines
A new way for a lot of you to do your brake bleeding from now on.
I know a lot of you are still doing the old fashioned way of having someone pump up the brakes while you bleed the wheel cylinders & keep repeating this process to no avail sometimes.
Get yourself a clear pop bottle or jar so you can see through it.
Get yourself 12-16 piece of rubber hose that fits snugly over the wheel cylinder bleeder nipple.
You might have to make it longer if needed
Loosen up the wheel cylinder bleeder but slightly unloosening & tightening it until it's loose. Don't just crank on it and break it.
Make sure the master cylinder if kept full during this process.
Fill the bottle or jar up half way with brake fluid.
Put the rubber hose on wheel cylinder bleeder nipple & the other end down into the brake fluid of bottle or jar.
Unloosen the wheel cylinder bleeder
Have some one pump up the brake pedal slowly & smoothly until all the air bubbles are gone & only brake fluid is coming through the line.
Tighten the Wheel cylinder bleeder
Do this to all 4 brake wheel cylinders & you will have no air in your brake system
THIS IS THE CORRECT WAY TO BLEED BRAKES WITHOUT USING ONE OF THOSE BRAKE BLEEDING MACHINES OR KITS THAT'S NEVER WORKED FOR ME
Try it next time & give me your feedback
Make sure you have plenty of brake pads in the front and brake shoes in the rear. Rear brakes shoes have an adjustment on them. They just might need to be adjusted up. Make sure the master cylinder is full of brake fluid. Make sure when you're checking the pads and rear shoes you look for anything wet or leaking. Lines, calipers, rear wheel cylinders. Along the frame rail from the front to the bad also.
Did you bleed the rear brakes with the ignition ON and having someone hold the brake pedal down to turn the pump on while you opened the bleeders? The front brakes can be bled the old fashion way.
After you are done bleeding, with the ignition off, pump the pedal to charge the accumulator. Around 15 to 20 pumps you should feel a change. This might fix the problem and I experienced this scenario over the weekend. Over on the NATO forum there's a very detailed and good write up on the procedure.