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I am rebuilding the vacuum tank on my 1929 640. Does the flapper valve on the inner tank face forward or to the rear? There is a locating pin on the main tank that limits the installation of the inner tank to one of those two ways.

Why does it matter? It simply holds the vacuum while the inner tank is filling and releases when it is full, it can face in any direction

Mount the inner tank and top cover where the location pin fitted, because it is to secure outside air to get access to the inner tank when the atmospheric valve opens at filled up inner tank. Check that the outside air passage is open.

Odd

Good to hear from you. The inner tank has 8 holes for the screws that attach the top. In addition, it has 2 slightly larger holes 180 degrees apart that go over a small vent tube in the main tank, effectively locating the inner tank's position. Placing one of those holes in the inner tank over that vent will face the flapper forward and vice versa. I don't know whether it makes a difference, just asking.

The position of the flapper valve could be in direction.

In this model vacuum tank there are eight equally spaced holes around the rim of the top. There is a ninth hole located in the rim of the top where the arrow in the picture is pointing. That mates up with a small post in the can, If there are more holes in the top, someone modified it. The large copper tube above the arrow is the vacuum supply tube. The small tube behind it is a vent tube. Its open end should not be near the exhaust pipe. The tube to the upper left is the fuel supply tube.

It should face forward has been my 50 year experience.

Several different answers, here. The picture is not mine, although mine is identical. Note the locating hole for positioning the inner tank. However, I agree with DavidM; It shouldn’t make a bit of difference which way the flapper faces.

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