These seats don't have springs. Stamped metal pans with a molded foam cushion.
86K10. Without seeing the seat, this is an edumacated guess. Sounds like the foam itself is dead. Compressed over the years to where it no longer has the memory to return to it's original form. Not only that, but due to the shape of the pan, as the foam has compressed, the outside edge has been broken down & cut by the outer formed metal edge.
The foam on the outer edge can be repaired, but it will always be a repair. It'll never quite as good as new, & certainly more susceptible to breaking down again.
No repair is going to last another 20+ years. Not even 1/2 of that. Be lucky to last 5 years. Maybe. Could be even shorter depending on the driver's size & how the seat's taken care of, along with how a person gets in-n-out of the truck.
Anyways, the seat has to be removed from the truck.
Undo the front bolts first. Slide the seat forward & remove the rears. This will allow the seat to stand upright on the bench by itself. Do that procedure in reverse & the seat will be flipping & falling all over the bench, fighting you the whole job. If it's electric, raise the seat up to it's highest position.
Flip the seat over. Around the perimeter are some long plastic clips sewn to the cover. Get a long flat screwdriver & flip them off of the frame's edge. The clips have little hooks molded in there that can be a PitA to get off of the frame's edge. Pushing down on the seat will compress the foam, giving you a bit of slack in the cover, allowing for a bit of an easier time to get it off.
Start from one side & work around to the other.
There's some velcro tags on the rear corners of the cushion cover.
Remove the rear edge of the cover last. That may or may not be attached with another plastic clip. IIRC, some are attached with Hog Rings. Twist the hog rings with a pair of side cutters to remove them. Cutting them is a last resort. If you have to do that, wear eye protection.
Pull the cover & the foam off as one.
Flip the cover's band off the foam so that cover is inside out. The faces are attached with velcro. Don't just yank the velcro apart. More than likely, doing that will also remove the one side of the velcro glued to the foam. Inch it off.
Now that you're down to just the foam bun, you'll be able to see where the frame has cut into the foam's outer edge.
Get that part of the foam that's broken down held apart with something. Couple pieces of wood or plastic.
Use yelow 'Automotive Trim Adhesive' (3M) to glue it back together. Spread as light of a coating of that as possible. Putty knives work well to smear it around. A light coat, as it will be absorbed by the foam. Let it dry without letting the parts being glued to touch. Hence the plastic/wood. 20 minutes to an hour. Need the glue to dry & form a skin so that a second coat of glue won't sink into the foam. Let the second layer of glue to dry about 5 minutes. You want it to be slightly tacky.
Slowly push the foam together so that it aligns as it was when new.
Now, inside that formed indented part of the foam where the frame cut into it, glue in some kind of tough material. Vinyl works well. Smear some glue in there. Some more on the fabric side of the vinyl. Stick the 2 together while still wet. Give it a good rub. This ensures that the glue mates to glue. Pull the vinyl off & let it dry. When dry, give a light coat to the vinyl & let dry till tacky. Roll that into that indent from one side to the other, forming it as you go. Give it a snip where there's wrinkles.
This ^^^ is to prevent the frame from cutting the foam again.
To build up the foam where it's collapsed, you're going to need a piece of foam slightly larger than the cushion. I'd say 1" thick of a medium/firm density. What's known in the trade as 4 pound foam. That's just the way the density is measured, not how much it weighs. Has to do with compression rates.
Set that on the underside of the foam cushion & trim it to the general shape. Better too big than too small. It can always be cut down some more. Kinda hard to make it bigger.
Place that on the seat pan. Put the foam cushion on top. Sit on the seat.
Trim and adjust the foam until you feel comfortable. You'll probably have to taper that added foam towards the rear of the cushion. An electric carving knife works well, but it can also be down with an exacto.
You may have to do several adjustments of that foam. Small ones at a time.
You're butt will tell when it's right. Do this as many times as you need to get the feel right. That new foam may be the size of the whole cushion, just the center or maybe just the edges. Take your time with this.
When that's done, glue that new foam onto the cushion underside. Don't need a complete coating. Just a few spots to tack it in place.
Put the foam back onto the frame. Attach the inside out cover to the velcro.
Pull that rear cover attachment through & under the backrest & attach it to the frame. If a plastic clip was used, no prob. If hog rings, where those were, you can use zip-ties. Doing the back first, ensures that the cover will be in the right spot for the next procedure.
Reach inside the inside out corner of the cover & while holding in place on the corner of the foam, flip the cover right-side-out, rolling it right down & over the frame. Do the other corner.
Turn the seat upside down. Pushing down on the frame compresses the foam & will give you enough slack in the cover to be able to snap those long plastic clips on the cover back onto the frame's edge. Re-attach the velcro at the bottom rear corners of the cover. (They wrap around to hide the hinges)
After all this, like I said, this is a repair. It'll never be the same as original. Close is as good as it gets.
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mariodave. The glued fabric covers can be removed. S-l-o-w-l-y-!!!
Peel the cover from the foam, using a razor knife where the foam is really stuck, working as if skinning a moose. (Canada here, eh?)
Steam helps a lot. It'll help to soften the glue. If the wife has a steamer for clothing, borrow that. (I won't tell!) Or...a kettle can be used too. Turn the seat upside down & let the steam work it's way in through the cover to the foam base. Don't worry about the foam getting wet. The steam won't saturate it & by the time toy're ready to install the new cover, it'll be dry.
See the previous for the re-n-re of the cover. It's virtually the same for a bench.