Sounds like you got air bubbles in
your coolant lines and it needs
burped. theres two ways to burp
your lines, take the cap off your
coolant reservoir which it sounds
like you already did, but in this case
since you have a brand new radiator
theres gonna be quite a bit of air in
your lines. So in this case the best
thing to do is take the Cap off your
coolant reservoir then you Locate the
thermostat and on top of it you
should see a small part that you
have to unscrew with an allen key.
All you have to do is unscrew that a
full turn then after that keep trying to
turn it over until it starts, once it does
go back around to the thermostat
and itll bubble for a bit burping the
air out of the system. Once straight
coolant with no bubbles starts
flowing out go ahead and screw the
nut back down. DONT TIGHTEN IT
DOWN TOO HARD it breaks easy.
Then let your truck run for 10 to 15
mins with the cap off to make sure
all the airs out. Hoope this helps