Most trans flush machines Are just a cylinder with a piston in the middle. You disconnect one of your trans hoses, and connect it to one side of the cylinder. The other side of the cylinder is connected to the other open end of the hose on the car. In one side of the cylinder, you have 16qts of new fluid. The old fluid goes into the other side, pushes on the piston, and new fluid goes into the trans. The exact amount that comes out, goes back in. It's about as good of a flush as you can do without disassembling the trans. But, you need to do 12-16 qts, as the new fluid mixes with the old fluid during the flush. It takes about 10-20 minutes depending.

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