I have a pair of Shimano XT ST-M738 Rapidfire Shifters which have been installed for a while now. I get the feeling that gear changing is getting stiffer although it could be my arthritic thumbs. I'd like to lubricate them but don't know if I should or how. I have looked for a service manual for this unit but can't find one. Can someone point me to a link or, perhaps, tell me how to do a service on these units, please?
Those are fantastic shifters. I've had them since they were released and still use them every day.
The bikes manufacturers of those days were fond of leaving open runs of cable in the middle of nowhere. In other words, the brake/gear cable will leave the shifter/brake inside a housing, go to a stop point, strip the housing, run naked, enter another short piece of housing and eventually end up at the end point. This is bad. Bad for braking, bad for shifting.
If this is the case with your bike, consider modifying the cable stops so that they can accept continuous housing. There is only one drawback and that is along the top tube you'll have a length of housing that's ideally short of one support. But, that's only for fussy people to worry about. Others put up with a bit of untidiness.
Secondly, treat yourself frequently with new cables - inners and outers. Don't skimp, be merciless. New cables make the brakes feel like heaven and shifters like heavenly beer.
If you don't work on cables every day like professionals do, you really don't know what an old cable feels like, but you can teach yourself. Here's how.
Disconnect the cable, brake or gear, at either end but leave it inside the housing. Now, using two hands, pull backwards and forwards on the inner cable, simulating operation. Feel the resistance and try to calibrate your mind. Now do the same with a new cable/housing combo. If you can feel the difference, it is time for new cables.
Or, you can simply wrap the cable twice around a large pole with a 100mm diameter and then operate both ends. This quickly shows you what a difference a new cable can make.
For reasons stated here in the past, it is best not to attempt to "service" old cables. They're internally damaged and oiling them is a short-term solution.
Then, those Rapidfires loooove WD40. Don't believe the naysayers. There is no such thing as a dirt magnet. The oil in there possesses neither magnetic, nor static properties. It works a charm in flushing out the rubbish and lubricates it the same time.