My truck has been running hot. I run the truck and the temp gauge runs all the way to the L in normal. When I press the breaks the gauge drops down to the normal temp. Is there anything I should know, dos or don'ts?

 

You need to figure out if it's telling you the truth or not. I'd actually suspect the sending unit first.

 

I replaced the thermostat, but spilled coolant everywhere haha. I put in the new one with a new gasket and more coolant. I ran the truck a few miles, and it slowly warmed up going down my driveway. With the old thermo it would had already been on the L by the time I hit the road. I ran her hard for atleast 5 miles and I could not get the gauge to go past the O in normal.