i have a hydroboost under the floor on my 58 truk.
it foamed for a bit but once the air was out it was good.
my truck has a mustII rack, i ran it full left, full right and left again
pressed the pedal to the max a couple of times
let it sit, filled it up and all was fine
new rack, used hydro off an 03 cobra
got to remember that the accumulator is similar to a bladder tank
full of air until it gets bled out
if it's pointed up hill at all it may take a bit to get the air out