Last night, after driving around with cylinder compression leaking into my
cooling system creating excessive pressure, I had a major head gasket
failure that was blowing coolant out like a firehose.  Being very pi##ed off
that my van is falling apart I was tempted to just drive the 20miles home
and see how effective mobil1 is in really extreme environments.  Anyway,
after cooling my jets for a few minutes I decided to try something:

Figuring I was developing high pressure (in excess of the 8-10psi that has
been reported as normal) in the cooling system because of the compression
leak, I pulled my coolant pressure cap and with long nose pliers and a
screwdriver I removed the pressure valve mechanism which then allowed
continual venting of air out of the cooling system.  I topped up the coolant
in the tank, put the cap on and partly unscrewed the cap on the overflow
tank to make sure it could bleed air out to the outside.

The result is that with no pressure able to develop in the cooling system as
it was allowed to vent to atmosphere, I was able to drive home with only
minimal coolant loss and temperature within the normal range (actually, much
more normal than when I was driving with a closed system and a compression
leak).  All the pressure and engine gas bubbles are allowed to escape, with
very little coolant escaping.  I was amazed that what appeared to be a
catastrophic gasket failure turned into just a tiny drip as long as no
pressure was behind it.

Hope this saves someone a night in the cold or an expensive tow someday.  I
would recommend frequently checking the coolant level however as even if it
stops your major leak, you will loose coolant due to evaporation or boiling
away as the system is no longer closed.

Mark