I think brake performance degrades so slowly that I don't notice it happening. It was forced into my attention this past week as a problem with rim tape had me changing multiples of tubes on my rear wheel over the past several days. During the process I managed to contaminate the rim with a soap like substance resulting in essentially zero rear brake. Almost stuffed into something before I got on the front.

I soaked then scrubbed the pads in rubbing alcohol but 4300 miles of use had given the wearing surface a glazed appearance. I scuffed them up using a wire wheel and reinstalled. I also wiped the rim with alcohol. First brake application this morning and WHOA!, figuratively and literally. It was extremely powerful, far more so than before I got the slime on the pads, probably like new. Have to be a little careful, especially on slick surfaces, since I can lock it up now.

I find that any methanol based chemical will clean up greasy and oily surfaces and as I have a cheap supply- that is what I use. That goes for brake blocks aswell and on the rim I use fine wet and dry paper.

But first time out and I am wary of those brakes-They work.

Your right it does degrade slowly, especially on rim brakes, which are not self adjusting, the space between the brake pad and the rim, should be as small as possible, this will grow as the brakes wear, if the wheel is slightly out of true, requiring a larger distance, then, get the wheels trued properly. Brake pads on bicycles are mostly rubber, which contains some Volatile Organic Compounds, and these will off gas, leaving the pads dry and hard, strong braking can exacerbate this. If the pads still have a lot of rubber on them, you can sand them to break the glaze, Pads for my Mountain bike are $2.50 a pair and the road bike pads are $4.50 a pair, frankly I find that it's easier to just put new pads on, when they get hard....