Spongy brake pedal

Check the rear shoes to see if they're out of adjustment and have to move too far. If okay lets bleed the rears this should take care of it here is a guide to help you.

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

Please run down this guide and report back.

Nope. Engine doesn't have to be running. More importantly, when a vehicle gets to be more than about a year old, never push the brake pedal all the way to the floor during bleeding. Crud and corrosion build up in the lower halves of the bores the pistons ride in. Pushing the brake pedal over halfway runs the lip seals over that crud and can rip them. That will cause a slowly slinking brake pedal but it might not show up for a few days.

If the pedal was good before this service, there is still air in the lines. If the reservoir ran empty, that air could be anywhere. You can get the air closest to the reservoir out by slowly pushing the pedal down halfway, (it should take about 15 seconds), then allowing it to snap back up quickly. Pushing it slowly sends brake fluid down but lets the air bubbles float back up. Releasing the pedal quickly washes some of the air back up into the reservoir with the returning brake fluid.

Were you able to get the bleeder screws loose on the wheel cylinders? If they're rusted tight, I have a trick for bleeding them.

If you have the truck up on jack stands with the rear axle hanging down, that will adjust the height-sensing proportioning valve to reduce brake fluid flow to the rear wheels to prevent rear-wheel lockup. That can stop fluid flow when you're gravity-bleeding. If you're pedal-bleeding with a helper, you may not get enough pressure to force fluid to flow past that valve. If you have rear-wheel anti-lock brakes, you may not have a height-sensing proportioning valve in the rear. Normally you don't have to do anything to bleed the ABS valve, but if the truck is not sitting level, there could be an air bubble stuck in it. I've never pinched a rubber brake line but doing so would block fluid from moving the rear shoes. An air bubble in the ABS valve could take just as much fluid so when you're not trying to fill both of those, the pedal might feel normal.

A brake pedal that's harder o push is due to lack of power assist. Raising engine speed under even a little load reduces vacuum. There's a check valve plugged into the brake booster, but the booster will only hold enough vacuum for two and perhaps three power assisted stops. After that you need to allow the engine to build up a higher vacuum again to recharge the booster.

If that check valve has failed, the booster won't be able to store any vacuum and the power brake assist will be in response to whatever the engine vacuum is at that time. To test for a leaking check valve, (or booster), observe the force needed to push the brake pedal when you're standing still. Stop the engine, wait a few seconds, then apply the brakes again. The pedal should be just as easy to push at least two times. By the fourth application you'll feel it get much harder to push.

Start by checking all fuses and relays highlighted.

Then, with a digital voltmeter, check continuity at the brake lamp switch.

Also let me know if you have trailer/towing package. If you do, then you will need to check that wiring as well.