I know there are other signs to look = for, but what does it mean when my 2.1 waterboxer ('86 Westy) holds lots of pressure = in the coolant system after running and sitting overnight in 20-degree = weather?  (When I cracked the pressure cap this morning, got quite a hiss, and = some air entering tank)  I strongly suspect
a bad head gasket.  Have a new = (Autozone) pressure cap.  I can only think of a few causes:
 
--It's normal for the system to hold = pressure like this (good hoses, huh?)
--Combustion gases pressurizing cooling = system (my guess)
--Bad pressure cap allowing too much = pressure to build in cooling system
--Too much air in system--the air would = act to "store" pressure better than a mostly-fluid system
    that wouldn't store = much pressure (because there is little room for = expansion/contraction).
 
BTW, this engine is going to be = replaced before long.  But I would like to understand what's going on
here.
 
Steve
'86 Westy