If your DM6 front on/off valve is leaking then it's due to the o-rings failing within the on/off assembly. It's actually a simple fix if you have spare o-rings, once repaired, you shouldn't ever run into that problem again if you don't touch the front on/off.
Refer to page 11 to see the DM6's on/off valve assembly: http://www.dyematrix.com/manuals/Dye...DM6_Manual.pdf
There's a reason why DYE got rid of the front on/off assembly in their later markers. If you want to fully eliminate this problem, then invest in a Custom Product Continuous flow plug as mentioned above.