Yea if you take the cap off does pressure come off? Your system should be under pressure!

Do your rads kick in and what voltage is supplied to the fans? Maybe wiring issue causing them to run low speed

Rad might be that the cooling ribs are not touching the core anymore when system is up to pressure making it really bad in cooling down

Also 3/4 is normal there the fan kicks in. If you go full scale on idle i would start worrying