No heat most of the time.

My F350 is having a heat issue. Most of the time it blows luke cool air, but from time to time it will go from luke warm to barely warm. Just enough to defog the windows, but not enough to defrost them. I have already done quite a bit of work to try to repair it, replaced a couple of marginal parts, but so far no luck getting heat. Here is what I have done and the results after each repair.

1. The first thing that I noticed was that I was not always getting air to blow from the proper ports, the blend door was acting up. I change the switch in the dash and the blend door started working.

2. I was also smelling antifreeze in the cabin, this is usually a sign that the heater core is plugged. I replaced the heater core. Problem is when I checked the old core with water, there was no blockage that I could find.

3. I noticed that the heater control valve was not working properly. It was stuck half way open. Installed new heater control valve (might explain why the AC seemed a bit warm). I noticed that there wasn't any antifreeze in the hoses when I changed this out.

4. Started wondering if I had an air lock. I was able to hook up a Air Lift system to try to pull out the air lock. However I was only able to pull 10 pounds of vacuum. System says it should be 24-26 pounds. I could not find any visible signs of a hole that would prevent it pulling a proper vacuum.

My heating system is a closed loop system. The hose routing starts on the passenger side of the engine and then goes to the heater control valve, then into the heater core, then along the back of the fire wall to the drivers side where it tees into the hose coming from the bottom of the reservoir and then back into the side of the engine.

In the end I have two questions:

1. What could possibly cause the system to only pull 10 pounds of vacuum? No visible holes, no coolant leaks.

2. Could my water pump be going out? The engine does not overheat.

Of course any other thoughts are welcome.
Thanks, Tim