Glen - Thanks for the clarification. Your pump is not the dual fuel/vacuum unit Otto was asking about. The hissing by your feet may actually be up under the dash. The hole through the firewall and vacuum hose into the cab is right above your feet, so it sounds like you perhaps have a cut/split vacuum hose.
Looking at your pictures again, the rubber hose through the firewall is passing through an unprotected metal hole. Could be rubbing against that has damaged the hose, or it may just be cracked/split from age. I would suggest disconnecting the rubber hose at the metal tube coming out of the intake manifold and capping off that tube. If the hissing stops, you've found the problem is somewhere in that rubber hose and/or up to the wiper motor.
