It sounds like a bad compressor valve.  Was this a retrofit from R-12?

Sounds like your compressor is completely shot. Are you sure your system
only takes 24 oz of R134? Because, unless it was less than 50 F outside,
the static pressure of the non-running system should be much higher,
about the same PSI as ambient degrees F, e.g., 60 PSI @ 60 F. You could
remove the compressor belt, apply battery voltage to the clutch terminal
and try to spin the compressor by hand. It should be difficult or impossible
if the compressor is working. If it spins freely when the clutch is engaged,
you need a new compressor.

By the way, as the clutch face wears, the air gap grows to where the magnet
is not strong enough to engage it consistently. A little corrosion on the
clutch
shaft only makes matters worse, which is why it doesn't work well after
sitting idle for 3 months.

It appears that there is more than 1 problem with your AC system.

-Multiple applications of sealer.
This is often due to an evaporator leak. Evaps require instrument
panel removal ($$$) to replace. These sealers basically don't work and
usually damage other parts of the system. People insist on "trying
them anyway, just in case" on the false hope that it will save an
expensive repair.  If one or more applications was a 2-part sealer,
the entire system is now trashed. Sealer is a HUGE no-no in an AC
system.

-Compressor not building pressure when engaged.
 Assuming there is any refrigerant to compress, Usually caused by a
broken compressor shaft due to continued high head pressure caused by
overcharging, sealer contamination, air in the system,condenser
blockage, condenser airflow problems....
Can also be caused by a missing orifice tube or the tube being blown
into the evaporator. You should notice some slight fluctuation in the
gauge readings in this case. No fluctuation means broken compressor
shaft.

-Compressor clutch not engaging unless it is "assisted"
  Clutch air gap too wide. Caused by normal wear, excessive head
pressure over an extended time, low current to the clutch coil, weak
clutch coil. If the gap is .025" or more, it's too wide. It can be
adjusted, but it appears your compressor is also damaged. Adjusting
the gap won't fix the compressor.

The only way to know the extent of the damage is to remove the orifice
tube and look at the debris. I can guarantee you will find steel
chunks, hardened sealer, pieces of burned Teflon ring material,
aluminum chunks or some combination of those. There may also be a
black gooey substance indicating a "Black Death" compressor failure.

Due to the sealer, age, and obviously poor prior repairs, your only
viable option is to replace the compressor, accumulator, condenser and
all underhood lines. (Firewall foreward replacement) Less than this is
flirting with problems and wasted money/effort down the road.

You also need to have the evaporator pressure tested to check for a
leak. This is the #1 reason for sealer use. If it leaks, figure
replacement costs into the price. If a 2 part sealer was used, it MUST
be replaced.

This is NOT going to be an inexpensive repair.
If the vehicle has a rear evaporator unit, the cost will be even
higher. Luckily I don't think the Expy ever had a rear unit, just some
duct work to the rear seat.

You can DIY the repair and save a good bit, but be wary of
"inexpensive" parts, especially the compressor. Your Explorer uses a
Nippondenso FS-10 compressor, so if the price is under $150, don't be
tempted. Cheaper FS-10 remans are very prone to shaft seal leaks and
other issues after about 1 year. (You'll be fixing it again $$$oon)
Good remans/new units are about $200-250.

I would estimate the DIY cost of the repair at about $750-800, using
decent quality parts. At a shop, it would be in the $1500 range. Many
shops won't even touch a vehicle with sealer contamination.

If you decide to tackle it yourself, check out www.autoacforum.com
first. Do some reading to get a handle on how to fix it correctly.

Good Luck!