The transmission flush is to clear the old transmission fluid remains in a motor vehicle. It enables to clear the majority of fluid and helps to remove the remains of oil transmission from coolers, converters and cooler lines. However, some questions arise as to whether or not transmission flush is necessary, but doing this process on regular basis, the life of transmission increases.
The process is to connect a drain plug so that old transmission fluid is removed properly and accumulated in a separate pan. Though the process is to cleanse the system thoroughly, but little bit of older fluid remains inside. So what happens is the older fluid gets merged with the new transmission fluid and gets diluted perfectly, becoming new fluid. The transmission flush is the only source that helps to remove most of the older fluid in order to cleanse the system. The less remains of fluid exist inside, the less contaminated the new transmission fluid will be. New transmission fluid means less stress is applied on the transmission while extending the life of the existing transmission.
It is not possible to drain the entire fluid out of the pan. In fact, the drain plug only lets around 3 out of 15 quarts. What happens is most of the quantity of fluid remains in the torque converter, cooler lines and transmission cooler which are the parts of transmission. Several vehicles including Mercedes have drain plug on torque converter, but few other manufacturers like Toyota do not provide this feature on their vehicles. However, many say that a flush can blow out the seals and can cause damage to the transmission. Well, this is not the real fact, so you can practice flushing of older fluids to maintain the quality of transmission. The process is to use pump that moves the new fluid and displaces the older fluid which is then collected and thrown away. There are basically two types of flush machines. The pump does not push the residue; it just displaces the older one with new.