I have a slightly strange issue with the heat coming out of the vents. There is always heating coming out but when I go uphill the heat really warms up. I was thinking I had an air lock. Not to sure if that is the issue. I am adding a little coolant every week or so. Had a bad leak at the water pump a year ago and replaced it. The pipes in and out of the core are hot and when you give them a squeeze you can see the fluid level in the rad respond. Thinking I may have a pin hole leak in the core but I don't see any puddles around the vehicle.
Any thoughts would be appreciated

 

Probably an air bubble in the cooling system, possibly due to the coolant leak. Have you checked the lower intake manifold gaskets for leaks? Also check for coolant in the oil, and a "milkshake" on the under side of the oil cap. Very common for intakes to leak on these, unless/until you install a Felpro MS98002T gasket set.