pressure in radiator after cooled down

[ddrive01]

Hey guys,

 

ive got a problem.

yesterday i started my bike after it has been sitting around for a month ( needed a new header which i got and installed yesterday).

Rode it for 15 -20 minutes on my block.

Today i wanted to check the coolant level.

After opening the radiator cap it still had pressure in it and some coolant burst out.

 

What could be the problem?

 

[ddrive01]

Oh and it did not spray from the overflow hose when riding yesterday