Bicycle Mechanics - What's the procedure to flush out STI levers with WD40?

I've seen doing this mentioned a couple of times, and it seems like this is something I need to try. I have 9sp Tiagra levers on my Novara Randonee, and I've been having a hard time getting the rear derailer to drop onto the smallest cog. I pulled, cleaned and reinstalled/readjusted the derailer, same problem. Then, I needed some other work done, so I added it to a list of things I needed done, and had my mechanic adjust it. He reports that just like I said, it's hesitant to drop onto that last cog, even though the limits, tension, and B screw are all set properly.

He said either the derailer or the lever is failing, and since he said that the derailer drops down OK if he operates it by hand, I figure the lever has a little shmutz up in it somewhere.

So, I have to retape these bars anyway, can somebody tell me where to spray into these works (and are there covers to remove?) while I have the tape off, to have a chance at flushing out whatever's causing my problem?

TIA

Did your mechanic not tell you that your problem could be excess friction in the cable housings?
As for WD40, I simply use the read straw that comes with a can of aerosol WD40 and shoot it directly into the shifter's internals. Won't hurt a thing and could help. Use plenty.
Another good thing to try is to shift into the smallest cog, disconnect the cable from the rear derailleur, slide the short housing forward on the cable exposing the part of the cable that runs through the short housing. Lubricate the cable with light oil (I use silicone grease). Reset the housing and attach the cable being careful to route it properly at the fixing screw. The short housing is often the location for dirt and corrosion causing excess friction and sluggish shifting.
It's also possible that your mechanic does not have the rear derailleur adjusted quite right. You could try turning the barrel adjuster (at the rear derailleur) 1/2 turn clockwise.

From your own description of the problem, you may have a cable tension problem. You do a flush when the shifter won't work at all, not just to one position.

So I flushed it all out, and it didn't make much, if any, difference. For what it's worth, I know how to adjust a RD, and I tried twice so I figured I'd let a pro handle it. He said pretty much what was in the OP, that without the cable attached, it shifted fine, but attach it and there you have the problem again.

I think the tension is correct, I've even tried making it far too lose, and still the same prob. For the other poster, yes new cables and housings, so this is not the issue. I think I may have made it a tiny bit better today by cleaning and packing some grease up into the upper pulley on the RD. I think I'll throw a NOS ebay derailer on this thing and see if that's the thing. Theory being that teh spring has lost it's pop.

The problem (as alluded to above but not explained) is likely a problem with cable/housing "FRICTION" along that rear shifter cable. If you've put lots of miles, or a decent number of wet miles on this bike, it might be a good time to replace the shift cables. I've heard it suggested that it's a good idea to replace ALL cables (shift AND brake) every two years.

Most likely the friction that's causing your problem is in the small loop of cable housing that goes from your chain stay to your dérailleur. Being that this section of cable/housing is close to the ground, it gets more water/dirt/grime thrown at it than any other section of cable, and often gets corroded or dirty inside, creating friction. When shifting to larger cogs on the cassette, your force against the shift lever is pulling the dérailleur directly. This is not the case on shifting to the smaller cogs, however, since a cable can only pull, not push. In order for your dérailleur to move the chain to smaller cogs, the springs in the actual derailleur need to be able to overcome any cable friction between itself and the shifter. If the friction is too great in the housing, then the shifts will be delayed or not happen at all. I'd bet that this is what's happening to your bike.

If you have pretty low miles on this bike, and don't feel like buying new shift cables, you can unbolt the cable from the rear derailleur, pull off the cable end, and pull it back through that rear loop of housing. You can inspect it for corrosion, and if it's in good shape, clean and re-lubricate the cable and housing and reinstall. Re-adjust your rear derailleur from scratch and give that a try. Good luck.

-Jeremy

Thanks Tunnelrat - but as above, the bike is sporting brand new cables and housings. After commuting today I'm starting to think that the RD is just worn some (it's a cheap one, and has about 3.5 yrs and 4500mi on it). There seems to be enough play that with the chain in the middle ring, I occasionally can't get it to drop into that last cog reliably (or quickly). On the big chain ring, this works fine, but there's some hesitation in shifting toward the bigger cogs toward the middle of the cluster. To me, this sounds like some slop developing. Of course open to any other ideas before I start checking ebay. ( It's a Mountain RD by the way, Deore LX 9sp if I recall correctly)

Does your shifter release or pull the cable correctly in both directions at your problem gear? If you remove the cable housing and hold tension on the inner cable you will be able too tell if it functioning correctly. I have never been a fan of wd40 for cleaning. Remove the front cover, and use Brake clean to flush the grease. It will dissolve everything and then dry clean. Follow with compressed air and then triflow . I did a ultegra 9 speed today that was hanging in both directions and it is great now. I am not convinced your shifter is your problem, isolate it to find out by removing outer cable and cycling the shifter. .