My initial suspicion would be that it's probably pretty low (thinking under 10psi). I need to read up on the steam vents. Are you running a block mounted automotive style water pump?
GT
Nope.
From the jet to gate valve
Gate valve to a tee
Tee into block
Out of block to a tee
One end of tee to a ball valve for the headers at my seat (closed all the time now)
Other end of tee overboard
Steam vents directly overboard