'85 Westy with 2.1L, using R-12

Only blowing around 80 degrees (better than 112, but not by much), mechanic
says 300lbs high side pressure and only about 10 lbs low side.  Says he
suspects a blockage "somewhere".  He drained the R12 and blew through the
condenser - says that's clear.

Due to my ingnorance of things AC, the hoses were initially hooked up
backwards at the compressor.  Mech says running the system in reverse may
have damaged the receiver/dryer, causing these symptoms.  Last night I
pulled out the expansion valve and am able to blow through it.  AC parts guy
says that means little, as it could be locked open or closed (this means
little or nothing to me [I feel like Rocket with this series of questions]).

I bought brass fittings last night with which to hook up a garden hose to
the system (sans compressor & receiver/dryer) to hopefully flow out any crud
hiding in the system, with the logic that any water inside will be boiled
off when a vacuum is pulled prior to recharging.  Anyone care to comment on
the wisdom of this procedure?

Hoping for cooler air,

Karl Wolz