I have Ultegra 3x9 shifters and derailleurs that were installed just a couple months ago. I purchased them on eBay and although they were new, they had been sitting in inventory for a couple years so not "new" in that sense. New cabling was used when they were installed.
Today my rear shifter suddenly stopped working for no apparent reason. When I attempt to shift up to a smaller cog, the entire brake lever moves with the trigger, so they basically cancel each other out. I inspected the cables and derailleur and everything seems fine. Shifting still works perfectly if I hold the brake lever so it doesn't move with the trigger, and downshifting to larger cogs works fine also. This tells me it's probably the shifter itself, not cabling or derailleur.
Googling around I find many posts advising the use of WD-40 to free up a sticky shifter. The idea is that the grease Shimano uses hardens with age and WD-40 will clean it out.
Is this likely the problem?
If so, how do I re-lubricate the shifter after the WD-40? To do it properly would require at least partially disassembling the shifter, which from what I've read and having seen the schematics, is something I really don't want to do.