Could be as simple as needing a new master cylinder,just because it has brake fluid dosent mean it isnt bleeding by internally and not functioning.
This is one of those brake jobs that if you have never done one before it is best if you find someone who has,these brakes are mechanical yes,but the pads and hardware are internal to the axle housing and the entire acle end needs to be removed from the truck to get to everything....it is also one of those jobs that you want done right the first time....meaning all parts and pieces replaced when the unit is apart......this includes brake shoes,wheel cylinders and all brake hardware....there is also a small brake line that goes to the wheel cylinder that is usually rotted and needs replacing as you wont get it off without breaking it...these brakes need to be done one side at a time...if you was to remove both axle ends there is a chance the cnter carrier can drop inside the drive unit causing more problems than its worth...the axle ends weigh upwards of 100 plus pounds and are very awkward to handle once free as the are not cnenterend and can turn unexpectedly and hurt or maime you....like i said if youve never done one before is best to have some one who has...also dont skimp on parts...better to do right the FIRST time...