Source: eHow

Flushing your own transmission can be an ideal way to extend its life and save money over the long run and is fairly easy to do if you’re at all mechanically inclined. Flushing the transmission is different from a simple fluid change, in that doing so replaces all of the worn transmission fluid that might be hiding in the torque converter or the many small passages in the transmission case. The procedure and types of fluid used are almost identical for every generation of automatic equipped Buick Century from 1973 to 2005, but you should skip the Type F for anything older.

What You’ll Need
1. You’re going to need two 1-gallon milk jugs, 8 quarts of Ford Type F fluid and at least 12 quarts of Dextron transmission fluid to perform a proper flush. You may also need 2 3-foot lengths of 3/8-inch rubber fuel line and the appropriate hose clamps.

The Lines
2. Identify the metal transmission cooler lines that go into your radiator. Front-wheel drive models have transmission cooler lines running from the passenger side front of the transmission, and rear-drive models have the lines running along the frame-rails on the passenger side of the engine and transmission.

Disconnect the rubber flex hose from the transmission-side of your cooler lines, and attach in its place the two lengths of rubber tube. Older models of rear-drive Century will have plenty of rubber line to work with, but front-drive models may require hose extensions. Have an assistant start the car briefly while you look underneath to identify which line is spurting fluid (the output line). Drop the spurting output line into one of you milk jugs. Fill the other jug up with the Type-F transmission fluid, and drop the other line into it through the top.

The Procedure
3. When you start the engine, the transmission will begin pulling the Type F from the supply jug and filling up the empty one. When the jug containing your old fluid fills up, shut the engine off, dump the old fluid into another container and refill the gallon jug with Type F. Repeat the procedure, and when the Type F is gone, temporarily reconnect your rubber lines to the radiator. Once they are secured, allow the engine to idle for 10 minutes. The Type F fluid contains powerful detergents that will clean out your transmission, but it will need to be flushed out afterward.

Repeat this procedure with the 12 quarts of Dextron fluid, reconnect everything, and you’re good to roll. This would also be a great time to replace the transmission fluid filter. Flushing should never be done to any transmission with more than 100,000 miles on it. The varnish inside may literally be the only thing holding your transmission together, and flushing will result in immediate failure requiring a complete rebuild to correct.