GE Profile Side by Side, about 20 yrs old.

It's maintaining temps but it has to run nearly non stop to do so.
Previously when I'd notice long runs it meant the condensor underneath
needed a good vacuuming & brushing. This unit always seemed sensitive to
that. But I've done that.

Also for background info, one time many years ago there seemed to be an
issue with sufficient cooling on the fridge side. (On this machine as with
many others, the freezer side runs the show in terms of the thermostat
directly controlling the refrigeration system while the fridge side is
controlled via a thermostatic bleed air vent.) I found the internal fan to
be sluggish and replaced it and the control board myself. Pretty annoying
to dismantle the evap area at the back of the freezer -- at one point one
must hang the evap on a cable to attend to this.

But I've never messed with the refrigerant circuit itself.

I know it's a sealed system and in theory there is no place for it to leak.
But who knows what 20 years of vibration can do. Also, to tell the truth
while it's a nice looking appliance, I was never impressed with the build
quality as there are refrigerant lines down in back that stick out so much
they are barely covered by the fiber board access & vent panel.

This is an R134a machine so I believe it's legal for me to charge it. The
full charge is VERY small...far less than even one can like I would use on
my car. Obviously there is no service valve.

So the question for you folks is whether it is worth buying a piercing
service valve (probably would put it on the service stub on the compressor)
and shoot in a TINY, TINY amount of gas and see if that helps before
calling a pro. Did I mention TINY?

Ancillary questions:

On HVAC, one can tell a lot about the state of things by measuring temp
drop across the evap coil. Is there any equivalent for a fridge?

What should the low side pressure be on a R134a refrigerator be? This may
be a completely different figure than for an AC system with the same
refrigerant.

Points if you recognize my handle without searching.