Retch

I am currently rebuilding a 15 year old road bike for my daughter, but am having trouble with the rear brake. It came standard with Tiagra 4600 levers and Tektro r312 brake calipers. When we got the bike, the rear brake didn’t have a lot of stopping power, so I tried a bunch of different things to fix it. I changed the cable and outer sheath, the pads, cleaned the calipers, lubed the levers and adjusted everything. It still didn’t work well, so I swapped in some other calipers I had spare, and that made no difference. The brake just has no feel or power. I can’t imagine it is the lever, but that seems like the only thing I have left to change! What am I missing??

Russ Roth

Using new cable and housing? Any kind of kink in either creates friction and changes how the cable moves, it can make the braking much worse. At 15 years old I'd also consider new pads, those can matter as well and make sure the pads are hitting flat. With older brakes, cleaning and lubing the pivots can help it move smoother. Also expect the rear brake to not function quite as well as the front, just a result of longer cables.

VegasTriker

You are not the only one with this issue about the brakes. From 2015 https://www.bikeforums.net/road-cycling/1032761 The recommendation seems to be not buying the OEM replacement but using Kool Stop brake pads. The only catch is that only Tiagara uses a pad with the holder molded into it while all the other Shimano models have separate holders and replaceable inserts. Check which you have and buy the suitable replacement. There is no doubt that the rubber has hardened over 15 years making the brakes far less useful for stopping. It's one of the first things I look to replace when refurbishing an old bike with rim brakes.

Schweinhund

Just from a generic point of view, hen confronted with a challenge such as yours, start from go. squeeze the lever, hard.
Is your housing moving when you squeeze the brake? compressionless housing might help.
Did you use good inner wires like Jag, or cheap Walmart inner wires? cheap ones can be stretchy

freeranger

You can purchase the pad holder from Kool Stop and when the pads wear out, can just slide in a new pad. Dura Road Holder Threaded – KOOLSTOP