WHY FORK SEALS LEAK
Fork seals, contrary to popular opinion, rarely blow!
Mostly they leak because they have a foriegn object lodged between the seal lip and fork leg, holding the seal open and allowing oil to leak out. Usually this is dirt or mud.
As modern forks grow in diameter to increase rigidity we also see an increase in seal drag due to the increased surface area. This additional drag creates unwanted harshness due to additional stiction.
In order to reduce the unwanted stiction the modern seals have less tension against the actual fork leg. As the forks flex the seals are stressed and any dirt on the fork tube may get passed the primary seal lip.
A common form of stress to the fork tube is caused by the break rotor, as you apply the break there is a rotational torque applied to the wheel opposing its natural momentum. The force creates stress to the fork leg which can stress the seals and create leaks. Hense the reason why most leaks occur on the break side first.
Another common problem is due to the forks bottoming and injecting the dirt sitting on the fork leg underneath the seals. Be sure to clean fork tubes between rides.
Last but not least, if you have seals that continually leak after repair and you are using OEM seals chances are your bushings are worn.
SOME USEFUL TIPS
Contrary to popular belief DO NOT try to clean fork seals with a feeler blade or the likes as this will often push the foriegn object into the fork where it can do real damage.
If riding on a muddy day ensure to clean fork tubes between rides. After finishing the days riding remove the dust seals and clean any foriegn matter with CRC and a soft rag then reinstall dust seal.
If the motorcylce has been sitting for more than a month make sure you lubricate fork tubes with a light oil before riding.