I nearly forgot I rebuilt some 80 series front brake calipers for someone a few weeks ago and then realised tonight I should have took pictures for the forum 
So what follows is a bit of a cheat because I took a boot / piston out of the finished job to take pictures of 
The main thing that seems to go wrong is the rubber boots round the end of the pistons get torn either through age or abuse and then mud sits inside there and corrodes the end of the piston so it won't go back in any more. Even if your boots seem pretty much ok it would be good PM to do a rebuild to make sure corrosion can't get started and cost you 8 pistons
Before you take the caliper off the truck, take the pads out and use the brake pedal to force the pistons out a good way. Once the caliper is off, clean all the mud off BEFORE to take the boots off or the pistons out.
Yours will be a dirty version of this:
They don't need to be spotless, just lose all the lose stuff for the bulk of the caliper and get them properly clean in the area of the pistons. Then pop the metal clips off and pull the boots off. Have another clean round the raised lip that the boot covered so the new boot has a clean surface to sit on. Now you can pull the pistons out and shouldn't get dirt inside the caliper.
This is one of the old pistons that really didn't want to come out even under braking pressure! You can see the marks from me putting the mole grips on very tight and slowly easing it out with a combination of tapping the grips with a hammer and rotating the piston.
The fluid in these calipers was particularly dirty and I suspect had a lot of water in it so there was even some small corrosion inside the caliper:
Here you can see the rubber main seal inside the caliper body. Just poke that out with a screwdriver. There will likely be some dirt in the recess that rubber sits in so give that a good clean as well - a rag pushed round with a small screwdriver works well. Then fit the new seal, it just pushes into place, and smear some of the special red grease that came in the rebuild kit on the seal so it's not dry. Then push the pistons into place, leaving quite a bit proud of the hole until the boot is fitted.
Pop the new boot on and go all the way round stretching it out slightly and looking to make sure the lip is sitting flat and not twisted. It needs to sit flush or this is all a waste of time!
Now push the piston all the way in, making sure you push it evenly and not too firmly. The pistons are a snug fit and only need to be slightly off square before they bind. If you're having too push hard then you may not be pushing it in as square as you think
Now the boot should easily stretch out over the outer lip. Make sure it's sitting evenly and flush. Then gently expand the metal ring till there's a small gap between the ends and put it over the outside of the boot. If the ring isn't sitting quite right, just expand it a little while pushing down gently where needed and it will go on easily. Make sure the ring is all the way on, flush against the rubber. It doesn't have to resist a great deal of force but you don't want it working it's way off. Avoid using anything sharp to adjust or poke the metal ring or you might puncture the boot - your fingers are best.
So what follows is a bit of a cheat because I took a boot / piston out of the finished job to take pictures of The main thing that seems to go wrong is the rubber boots round the end of the pistons get torn either through age or abuse and then mud sits inside there and corrodes the end of the piston so it won't go back in any more. Even if your boots seem pretty much ok it would be good PM to do a rebuild to make sure corrosion can't get started and cost you 8 pistons
Before you take the caliper off the truck, take the pads out and use the brake pedal to force the pistons out a good way. Once the caliper is off, clean all the mud off BEFORE to take the boots off or the pistons out.
Yours will be a dirty version of this:
They don't need to be spotless, just lose all the lose stuff for the bulk of the caliper and get them properly clean in the area of the pistons. Then pop the metal clips off and pull the boots off. Have another clean round the raised lip that the boot covered so the new boot has a clean surface to sit on. Now you can pull the pistons out and shouldn't get dirt inside the caliper.
This is one of the old pistons that really didn't want to come out even under braking pressure! You can see the marks from me putting the mole grips on very tight and slowly easing it out with a combination of tapping the grips with a hammer and rotating the piston.
The fluid in these calipers was particularly dirty and I suspect had a lot of water in it so there was even some small corrosion inside the caliper:
Here you can see the rubber main seal inside the caliper body. Just poke that out with a screwdriver. There will likely be some dirt in the recess that rubber sits in so give that a good clean as well - a rag pushed round with a small screwdriver works well. Then fit the new seal, it just pushes into place, and smear some of the special red grease that came in the rebuild kit on the seal so it's not dry. Then push the pistons into place, leaving quite a bit proud of the hole until the boot is fitted.
Pop the new boot on and go all the way round stretching it out slightly and looking to make sure the lip is sitting flat and not twisted. It needs to sit flush or this is all a waste of time!
Now push the piston all the way in, making sure you push it evenly and not too firmly. The pistons are a snug fit and only need to be slightly off square before they bind. If you're having too push hard then you may not be pushing it in as square as you think
Now the boot should easily stretch out over the outer lip. Make sure it's sitting evenly and flush. Then gently expand the metal ring till there's a small gap between the ends and put it over the outside of the boot. If the ring isn't sitting quite right, just expand it a little while pushing down gently where needed and it will go on easily. Make sure the ring is all the way on, flush against the rubber. It doesn't have to resist a great deal of force but you don't want it working it's way off. Avoid using anything sharp to adjust or poke the metal ring or you might puncture the boot - your fingers are best.
or pipe clamps are cheap and stick a plastic bag over the end wrap it round lots and zip tie it if you're trying to keep it clean.
