Your truck is probably suffering from stretched / frozen rear cables,
and possibly rear shoes that are worn out / way out of adjustment.

The adjustment point is found on the front parking brake cable
underneath the vehicle.  Find the point that the parking brake
mechanism attaches to the floorboard area and look for a black rubber
or spiral steel covered cable coming out from there.  Follow that cable
until you see a "Y" bracket where it attaches to the two rear cables.
Right there you will find a threaded shaft and adjusting nut.  It may
be frozen badly with corrosion.

A good plan of attack for a 1985 S10 would be to check/replace the
shoes and wheel cylinders, clean the backing plate contact points of
rust, replace both rear cables, and replace the front cable if the
adjuster stud is beyond repair.  This is assuming that none of these
repairs have been made for a long time.  They do frequently get
overlooked as many people don't use the parking brake or use it
sparingly.

Toyota MDT in MO

The adjuster for the emergency brake is at the back brakes.  You need to
inspect the rear shoes and replace if needed, then you need to adjust
them.

Once adjusted, then the emergency brake can be set up.  Normally the
emergency only needs to be set up when you put new cables in.  The rear
shoe adjusters look after the wear.  When new shoes go in, it is
unlikely the emergency needs the cables adjusted unless they have
stretched which means they will fail soon from rust/rot, especially if
you are in the rust belt.  

If the cables all still move, there is a nut on the primary cable from
the front where it splits into the two cables for the back sides.  You
adjust this until the pedal grabs 3 or 4 clicks down to set things up.
I recommend you start spraying this nut several times over a couple days
with a good penetrating oil before starting to work on it.

By the time the emergency brake stops grabbing with the pedal all the
way down, the back brakes are worn out normally.  Put new shoes in and
adjust them and the pedal grab spot comes back up high.  

Note:  If you adjust the cable so the pedal grabs and the rear brakes
are not properly adjusted first, the cable will become a pivot point for
the brake shoes.  This will cause cable failure pretty quickly and could
cause brake failure if parts start falling out.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
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what the other guys said.

Having owned an 86 S15 Jimmy, here's what I'd check:

-Jack up back end, pull drums. (and tires... ;)
-if brake shoes are worn, replace them first.
-check the rear brakes are properly adjusted - I had a stripped and
seized adjuster on mine.
-reinstall drums, but leave tires off.
-apply ebrake 3 or 4 clicks.  The drums should be hard to turn by hand.
-to adjust, it's like Mike said, but if it's like mine, the whole bloody
thing will probably be one big lump of rust.  I ended up using a dremel
to cut the cables all off and replaced them all because they were ALL
seized up except the one from the brake pedal.  IIRC there's actually 5
separate parts - a cross cable, two "intermediate" cables and the two
cables that go into the drum assembly.  The intermediate cables are
different between the SUV and pickup (and I imagine between a regular
cab and an extended cab too.)
-once you've replaced all the cables and adjusted them, put the tires
back on.  :)

Seriously, the problem is probably the intermediate cables or the other
ones being all rusted up.

Ray