Fuel it up again Dummy
The mix percentage has a affect on the temperature the coolant will reach. Water has a better heat transfer then antifreeze so it will act differently. So if you lower amount of coolant and add more water the truck will run cooler and that will lower the pressure inside the cooling system. It is all relative but if your not in a real cold zone you can dump a gallon or more and add straight distilled to bring level back up. Your pressure will drop 1-2 psi, I know this for fact because I have done it myself.I did ask if the coolant mix is off a little if it would cause higher pressures, but I was told no. I think it might though because the boiling point is different with different coolant/h20 ratios and I am sure mine is not exactly 50/50. I was leaking coolant a while back somewhere around the fan shroud but I know it wasn't the water pump; anyways I kept having to add coolant/h20 while it was leaking and out of the blue it just stopped leaking! So the ratio might be a little off. I am not monitoring ECT/EOT right now but it is next on my list.
Another thing if you lower coolant level you will have more air so you will have more area for thermal expansion and lowering the cooling system pressure. The important thing here really is: once heated up to operating temp if you vent the cap does it build back up or not!?