Sounds like you have an air pocket behind the thermostat. Drill about a 3/16 hole in the flange of the t-stat to let the air out as you fill the system. It also helps to park it nose up on a steep grade to help bleed the air out of the system.
Good luck,
Scott

Scott Ward

2 1948 1.5-ton Loadmasters
The red one and The snow pusher
1 1950 3100
1 1955.1 Chevy 6500
1 1954 Chevy 6500 2-Ton
1 1955 1st Series COE 5700
1 1963 K20 (454)
1 1964 C10 (350)
1 1951 1.5-ton Dump Truck
1953 and a 1956 Ford F800

Raising a teenager is like trying to nail Jello to a tree!