Reciever/dryer does what it is named. It recieves and stores the liquid
refrigerant that has condensed from a gas, to a liquid, in the
condenser.Like water vapor that rises and cools. It becomes rain.In this
case, the rain is (hopefully) r-12.It will puddle in the reciever and be
avaliable for use.

Inside the reciever/dryer, is a bag of dissicant.Same as in the box for the
new camera,or electronic equipmeent, that you bought It will absorb any
moisture that gets in the a/c system.  I think it will hold about 20 grams
of water.

 1/20th of 1 drop of water will shut a a/c system down. Any moisture will
circulate with the refrigerant and oil. When it  goes through the tiny
orifice in the expansion valve, it will collect as newly formed ice.The
refrigerant changes tempature from about 150 degrees, to about 35 degrees
in a flash, because the pressure has been lowered when it  passed through
the orifice. This ice will accumulate and block the orifice passage.
Cooling  will stop,and the a/c system will warm up. This ice will melt and
the process will start again. Intermittant cooling is the symptom.

When a a/c system is turned off, the pressures balance to about 75psi.
Moisture cannot enter. Moisture can enter a  system in the low side, when
the a/c system is operating. Pressure in that side can be at a vaccum.
Atmosphere will pressurize air and humidity through any leak.Some leaks
only leak in one direction.NOt bouth in and out,This moisture will collect
in the dissicant bag. When the bag gets old, it sometimes disentegrates and
you need a new one.

Teh vanagon reciever/dryer also has a sight glass for charging. When the
bubbles disappear in the glass, the system is full.