Why is my bmw e36 316 top hose building up lots of pressure?

i recently change my top thermostat housing and the main thermostat . i then bleed the system and im getting heat through so i know there is not an air block, the Temperature stays perfectly in the middle and the car has not overheated, but the top hose seems to build with quite a bit of pressure, and i have no idea how to fix that?Why is my bmw e36 316 top hose building up lots of pressure?All details are on their home page