This is the reason you bench bleed. First off did you replace or rebuild the master cyl.? Take the tube loose from the back of the master cyl. Hold a finger over the hole & pump the pedal from the back position to a little way down. Keep doing this till you get pressure on the outlet & no bubbles come up through the reservoir. Then hook up the tube & bleed at the wheels. Speedy bleeders will help or use a helper. When the helper pushes the pedal down close the bleeder & the helper lets the pedal back.I had a pressure bleeder for years & never used it. I have a sucksion type now & have never used it for brake bleeding. You must have a good fresh master cyl. though. If the tiny hole under the little funnel in the resorvoir is stopped up it will never bleed right. When you get the brakes right on these trucks there is barely any movement in the brake pedal. You just apply pressure.
