[castongu]

Sounds like those lines coming out of the firewall are for your AC system. I only have a '73 service manual to go by, but if the '75 models use the same color codes, Black Plastic line goes to Manifold Vacuum at the rear of the carb, the Grey Plastic line goes to the Hot Water Valve on the heater hose, and the Violet line goes to the Vacuum Reserve Tank ("round thing"). If your hearing a hissing sound under the dash you might have a hose disconnected on one of the Vacuum Actuators for the air blend doors on the heater box, or a spit hose. Could also be the connection at the rear of the Heater Control panel. There's a Rotary Dial on the rear that moves with the position of the lever. Post a few pic's when you have a chance.