overheating again

[mexlokote]

i recently fixed a leak on my inline six 250 4.1
the leak was coming from the thermostat housing neck,
so i put everything back together with new v-velts
i turned the engine on and left it running for 10 minutes
i checked the radiator and hoses if it was overheating because my thermostat guage i dnt think it works its aways on cold, (but has new thermostat)
but the radiator and hoses were cold so i though thats good, then i turned the engine off, one minute later the radiator and hoses started to get really hot,
the upper thermostat hose that goes to the radiator was leaking water with a lot of preassure really hot water


any ideas?