Transmission leaks occur for several reasons. Below you'll find more about the cause of those leaks, and some ways to stop them. The best way of all is to physically replace the leaking seals, but it is usually an expensive repair job.
Reason 1- Acid impregnates rubbery seals, and this causes seals to shrink, harden, and leak.
There is lots of sulfur in transmission fluid - and motor oil, for that matter. Some of the additives in those fluids are there to neutralize acids that form; from humidity that collects in among the fluid and oils. Moisture, plus sulfur, plus heat makes sulfuric acid - which, with heat tightens the rubbery stuff in seals - causing the leak.
Reason 2- Seal hardening wears a grove in the shaft it is surrounding. This causes a space for fluid to leak out.
Those are the two reasons and causes of engine, transmission, a/c compressors, water pumps, axles, and power steering seal leaks.
Most Stop leaks work because the solvent - in the additive swells the seal, and the leak stops. The swelling may not stop!
There are two bad things with solvents in any fluid or oil:
1- They make the seal gummy and weak. This often restarts the leak as the seal
disintegrates - or the solvent evaporates - and shrinking occurs again. There are
internal seals - in engines, transmissions and power steering units. It is not a good ideal to make those seals spongy. They need to be tough.
2- Solvents destroy lubricity. This increases friction and friction drag - and triples the
wear out speed. That does not sound to good for the engine or transmission to
me! Those are reasons why you should not trust solvent-type stop leaks. You can tell they are solvents by the look of being clear, and very watery - not oily.
Better Stop leaks.
Several companies make good 'stop leak' products that do not contain solvents. They work by neutralizing acids and by an ingredient called a friction modifier. These ingredients produce a controlled swell and stop seal and gasket leaks permanently.
Friction Modifiers also condition piston, bearing, valves, and gear surfaces by changing them from wear-roughen, and fast wearing, to smooth and slow wearing. That's good!
Some people have used a brand of stop leaks is not a solvent and also contains friction modifiers. They have a beneficial lubricating value to restore a seals elasticity. You can learn more about products to stop steering, coolant, engine, transmission, and axle leaks at the Related link shown below.