The little sediment bowl on the bottom of the T gas tank is one of the most important parts of the car. It can actually make the difference between a smooth-running engine and one that keeps missing.
What exactly is the purpose of this part? The obvious one is to provide a shutoff for the fuel when either parking the car or working on the fuel line or the carburetor. The more important function, however, is to be a fuel filter and keep the 60-year-old crud in the gas tank from entering the carburetor and fouling things up. The bulb shape provides a convenient place for foreign particles to settle, later to be drained off with the petcock at the bottom. Water, a common ingredient of gasoline in the '20's, can sink to the bottom of the tank, and also collect here. The large nut at the front of the bulb gives access to the fine screen which acts as the filter, keeping whatever sediment there is from flowing down the gas line.
To properly rebuild this unit, it is necessary to disassemble it. Herein lies most of the problems. It is not at all uncommon to see bulbs with broken shutoff handles and the corners of the large nut rounded off. When the part doesn't move, a lot of people take a bigger wrench and end up twisting it off. The secret is not so much leverage as it is patience and temperature, and the right tools.
In removing the shutoff, take the cotter pin, spring and washers off the end opposite the handle. Instead of pounding on the small-diameter end of the shutoff, use a socket from a socket set and fit it over the small end and up against the body of the shutoff. I find that a Craftsman 1/4-inch drive, 9/32-inch deep socket will fit over the end and is small enough to enter the body of the bulb. Tap on the end of this socket and the shutoff should come out. If it does not, don't use a bigger hammer. Use temperature.*
The sediment bulbs are made of various combinations of brass body/brass shutoff; steel body/brass shutoff; and steel body/steel shutoff. When parts are tough to move, many people try heat to expand one of the parts and loosen them. On pieces where the holes are much smaller than the main part, applying heat can cause the hole to shrink and lock it in that much tighter. Use just the opposite and stick the part in your freezer for a few hours. This should cause enough shrinkage of parts to loosen them up so that can be easily tapped out.
The same idea applies to the large nut. First use an 8-point socket of about 11/16-inch to fit over the nut flats. If it doesn't move, put it in the freezer and try again later. The large nut has always been brass on the bulbs I have seen and this will shrink more than the steel and cause it to loosen.
After getting everything apart, clean everything and replace the filter screen with brass cloth available at most hardware stores. Tighten the large nut pretty tight after first rubbing the threads with soap to keep them from seizing. Rub soap on the shutoff handle tapered part to keep it from seizing and to keep the gasoline from leaking. Assemble with washers, spring and cotter pin.
When using the shutoff, lightly tap the handle end to seat the valve. Before turning, lightly tap the other end to unseat it. The handle can then be turned by hand without the use of a wrench. Drain the sediment bulb frequently. I have seen cars with the bulb so full it completely blocked the screen and shut off the fuel supply. If done right, a good bulb will keep dirt and such out of the carburetor and will truly make the difference between a smooth-running engine and one that is not.
* NOTE: The bulb MUST be removed from the fuel tank, not only for convenience but also for safety. It should be thoroughly flushed out with water before using any sort of flame to heat it.