? Cooling system pressure?

[oldiesel]

From time to time on here i read that if the pressure in you cooling system keeps the hoses hard a long time(overnight) that it is an indication of a blown headgasket or even a cracked head.I dont understand how this could be,my understanding of the way pressure is in a cooling system,that with a cold engine there is little or no pressure in the system and after you start the engine the temperature slowly builds up as the heat causes expansion,the temp continues to build until the thermostat opens and allows coolant flow thru the radiator,if at any time the pressure exceeds the rating of the pressure cap the pressure is relieved down to the design pressure.
When after reaching normal operating temp the engine is shut off the temp will briefly rise as the coolant is no longer circulating after this brief rise the engine gradually cools down eventually reaching ambient temp.Cooling temp of course reduces pressure to the point where there is little or no pressure and in some case the pressure will drop to the point of collapsing the hoses, so to avoid this most modern cooling system caps include a Vacuum valve to allow some air to come in to balance the system to atmospheric to avoid collapsing hoses.
Now my question is what happens with a blown headgasket or cracked head that prevents the pressure dropping along with the temp as the engine cools down? Would seem to me that the pressure would drop faster once the engine was no longer running as the pressure would have an additional avenue of escape thru the blown head gasket or crack in the head, holding pressure makes no sense to me at all,if you have a theory please enlighten me.
Sorry for the long post Don

[Diesel911]

I am going to make a guess.

The assumption is that when you have a Head Gasket leak there is a pocket/s of gas somewhere in the system. So since gas is compressible even if the gas pressure exceeded the Radiator Pressure Cap pressure and vented off some that would still be compressed gas in the system.
When the Engine has cooled off the gas volume has decreased but is still pressurized and I believe it would hold that Vacuum Valve in the Radiator Cap closed. So in this case your Hoses would remain inflated; so to speak.

So the compressible gas volume acts differently than the uncompressible Coolant.

When the Coolant volume decreases it opens the Vacuum Valve and decrease the system pressure.

However, a low Coolant level that has an Air Pocket may be able to mimic a Head Gasket problem by haveing the had Radiator hose.