Just for the good of the order, a wordier explanation-

When the car is hot, the coolant expands and the system pressure increases. When it exceeds 15-16 psi, or so, the pressure cap releases coolant to the recovery tank, thereby limiting the system pressure and preventing “unscheduled rapid release of coolant.” (Pressurizing the system has the desirable effect of raising the boiling point of the coolant.)

This pressure makes the hoses feel firm.

When the car is turned off, the engine stops making a waste heat, the coolant contracts, and the pressure drops. At some point during the cooling, the pressure goes slightly negative. The expelled coolant was routed to the bottom of the recovery tank. This is important. Now, atmospheric pressure can push the amount of coolant expelled into the recovery tank back into the circulating system. This function makes it also important that the coolant recovery route is leak-free all the way to the bottom of the tank.

Now we’re at 0 psi, and the hoses feel soft.

Rinse, lather, repeat.

 

Great detail, I always just thought coolant was sucked back into system from recovery tank by vacuum..

Thank You Sir

 

Pressure differential results in vacuum on the low side in a closed system

PulpFriction's explanation did not get into why it is important to not over fill the recovery tank, but it is implied. There is a full line on the bottle, it is hard to see.

 

10-4 Thanks Don..

 

You are correct, really. Except in physics class. In that world, basic instruction emphasizes that sucking really means higher pressure pushing stuff to a place of lower pressure. “Vacuum” is just absence of pressure, and can’t do anything unless there’s pressure somewhere else.

Good point! Overfilling if nothing else will, upon warmup, spill excess coolant to the ground.

 

Thank You ALL