When I changed the rad last year, I filled up the overflow tank to the hot MAX line. Now the hot level's about 1/4 below MAX. Is it normal for air to bleed out the system over time and slightly drop the level like this? Don't see any leaks anywhere.

 

It might be your water pump.

 

It might be air bleeding out and it might be another leak. Top up the tank and keep an eye on it.

 

air is going out. coolant is getting in.
No leaks but coolant still goes down it's the water pump.
Major wear acceleration. make sure that's it, and change it asap.

 

How can you tell for sure its the water pump tho? Is it an expensive repair?