Road Runner Nest

I was working on my 71' Roadrunner tonight and I sat here scratching my head over this canister. I'm guessing this is a vacuum canister for something. The car was built in california. Problem is my car is an original 4 speed with manual HD drum brakes. So what the heck would this thing be for. It is located under the battery tray. I know the that the master cylinder in the pic is not original. That's cause I've got the original in a box while this replacement is used just to give the car braking power.

Your car has A/C and that's a vacuum amplifier for the water valve. Someone here might have a pic of the hose system from the service manual.

Technically it's a vacuum reservoir, it provides a more consistent level of vacuum for the all the A/C controls regardless of fluctuations at the intake manifold during engine operation. I only correct the term because in later years there is another device that was called a vacuum amplifier used for emission control systems. The same basic reservoir was used for other systems as well. air grabber, Dodge/Chrysler headlamp doors to name a few. The mounting brackets and nipple sizes varied some.
It seems the more actuators/controls involved in the A/C system determined the need for a reservoir. In 72' the water valve was cable controlled and did not use a vacuum reservoir for the A/C, even with basically the same heater/AC box, the vacuum take-off is connected directly to the intake manifold.

See them offered on e-bay all time as "Air Grabber can", commonly have the wrong brackets however.

here's the same can with a two bracket configuration, headlamp door system I believe.

So 72RoadRunnerGTX, these AC cans are swappable with the AirGrabber vac can? The only reason I ask is I have AG issues, and was gling to swap out the can b4 swapping out the actuator mounted on the hood. Thanks.

The only difference between the reservoir pictured above for the A/C and the one used for the A/G is the bracket is shaped to fit the contour of the inner fender under the battery box. If carful, brackets can be swapped; small tabs tend to break off however. The reservoir is a pretty simple device, empty sealed can with two nipples, one has a check valve. The nipple with the check valve (un-marked and off to one side of the center) needs to go to the vacuum source (manifold fitting). The other nipple is located directly in the middle and identified with a green paint dot connects to the center position of the switch with a green striped hose. If there are no leaks in the A/G system, it provides enough vacuum reserve to open and close the A/G two or three times with the key on and motor just turned off.

As with any vacuum controlled devices/systems, leaks are the main source of problems. A hand-held vacuum pump is the best tool to locate any leaks. The A/G switch tends to be the likely spot for leaks. Make sure you have the solenoid valve plumed correctly, on the A/G up line, and functional as it is intended to be a leak when not energized(key-off).