If the bleeder screw is on the bottom, you have the caliper on the wrong side. There's no way air is going to go down and brake fluid go up to get the air out.
I'm the master at getting the last ounce of life out of anything, but I draw the line at used brake parts. There's way too many variables that will result in unbalanced braking power side-to-side. Back in the '80s when we commonly rebuilt calipers and wheel cylinders as a part of a standard brake job, a replacement caliper piston could cost $20.00, the rebuild kit another $10.00, then you still had an hour of work to do the job. Today professionally-rebuilt calipers are extremely inexpensive, so there's no value in using used parts.
Depending on the model of the truck, and the year, there were also calipers with 80mm pistons and with 88mm pistons. Both sizes were available as standard or a low-drag version. That means there's four different calipers that can be bolted onto one corner of your truck. There's a 32-page service bulletin to address a very sudden, and very hard brake pull to one side that shows up unexpectedly after a number of hard stops in a row. One of the things we had to check was that the calipers were matching.
If you're determined to use the wrong caliper, there's two ways to get the air out. One is to unbolt it, hold it with the bleeder screw up, then let it gravity-bleed until the air bubbles stop showing up. Another way is to leave it bolted in place, then loosen the banjo bolt that holds the flexible rubber hose onto it. Tighten the bolt when air bubbles stop coming out.
Given the age of the truck, (more than a year old), if you've been running the brake pedal all the way to the floor, it is likely the master cylinder has also been damaged. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Pushing the brake pedal to the floor runs the rubber lip seals over that crud and can rip them. That results in a slowly-sinking brake pedal, and that often takes two or three days to show up. This is another place where salvage yard parts are a very poor value. Any replacement master cylinder will have to be bench-bled before it is installed, and that can result in the same problem.
As a brake system specialist, I have only used gravity-bleeding since the late '80s. For those who use pedal-bleeding, the helper must be instructed to never push the brake pedal over half way to the floor.
You can find more information in these articles:
Here's links to some related videos too. This first one is on bleeding brakes:
This second one shows how to bench-bleed a master cylinder:
Please keep me updated on your progress.
Wednesday, April 7th, 2021 AT 4:43 PM