Big Dawg

Hey guys quick question in regards to this black canister on the firewall. What is the purpose of it? Also can someone post a picture of it and where the vaccum lines are connected to it and where they come from.

thanks

Fun71

It's the vacuum reservoir for the HVAC system.
One vacuum line (larger diameter, maybe 1/4"?) attaches to a nipple on the intake manifold behind the carburetor.
A second line (smaller diameter, maybe 1/8") goes from the reservoir through a hole in the firewall to the HVAC control switch.

joe_padavano

The various actuators in the A/C system are controlled using engine vacuum. These actuators operate flapper doors in the HVAC box that direct airflow. Manifold vacuum goes to zero as you open the throttle, meaning that when you step on the gas, there is no longer any vacuum to hold these HVAC actuators in position. As an example, if you had the A/C on and were climbing a long hill, the airflow out of the dash vents would stop as the vacuum dropped. That tank serves as a reservoir to provide vacuum during these low vacuum periods to allow the system to continue to operate. There should either be a check valve built into the tank (if it has two ports) or an external check valve in the hose leading to the tank (if it has one port).

oldspackrat

It was also used if your 1969 or older Oldsmobile had the power trunk option. In 1969 & older cars that option used a vacuum operated solenoid located in the rear of the trunk. This vacuum bulb provided at least one more chance to open the trunk when & if you turned the car off. I have seen 2 of these bulbs on cars that had AC & power trunk but IIRC they were all big cars & not A body cars.

joe_padavano

Also on cars with vacuum door locks, for the same reason - the vacuum reservoir allowed these actuators to operate when the engine wasn't running and thus there was no manifold vacuum.

Don R.

My 63 Starfire with A/C and vacuum trunk release has two of these mounted on the firewall plus a giant metal vacuum tank under the left fender for the power brakes. I resealed these with RTV to fix some A/C control issues.