how does air get in the cooling system?

 

many ways, 1. no coolant in resevoir causes system to suck air when cap opens, 2. leaky headgasket forces air in, 3. leak in systems allows air to replace coolant, 4. faulty cap, 5. the most common, air in system after a repair that wasn't properly bled.

 

add one more to that......cracked sleeves. i think i just found this out the hard way.