1982 c-60 brake help

[sweetk30]

1982 c60 drum/drum man master on cab / vac /master down under cab . bbc power stick shift. non 2 speed rear.

friend just got this truck and its hissing when brakes applied under the cab .

its a man booster single line on firewall down to a vac booster master under cab .

its hissing from the vent/filter cab that comes from a 3/4inch hose off the black plastic unit off the master unit.

also I think the brakes need to be bleed out as the pedal is low and stays up if you pump it 2-3 times. I did check and the adjusters are up good and close so the shoes are about there on the drums .