Worn transmissions will slip if you flush the old fluid out. This is because the old fluid contains bits and pieces of the memorable clutch, which increases the friction capacity of the fluid. With a worn clutch, more energy is transferred between engine and transmission. Over time, this increased viscosity and energy will increase the pressure in the veins of the machine, the clutch fragments sanding down the walls, and eventually, a seal will blow out, and the life fluid will spray everywhere until the machine grinds to a deathly halt. The only way to fix this is to completely rebuild the transmission one piece at a time, and put new fluid in. Cost and time wise, it’s better to just try another transmission. But people are more than a transmission. Sometimes, we get too emotionally involved, and we dig a hole to the center of the earth. A transmission cannot repair itself; it needs the help of a mindful mechanic.