There's your trouble. Yer doin' it wrong. Yeah....any state that can thermo just can't be trusted man.
Not a clutch fan, rad cap is new, always buy new t-stat & never reuse old, using straight water for now til I know I'm done messing with cooling system, then will replace w/ 50/50, never heard of t-stat vent hole...I will look to see if I have one (what's its purpose?)