Contaminated pads typically won’t slow the bike down very well. They may also squeak wildly.
Contamination can come from leaking hydraulic brake lines on the bike, from sloppy brake bleeding, poor maintenance procedures (over-spraying oil on your chain), or from outside sources such as riding through a puddle that has an oil film on it. Even handling the pads with greasy fingers can lead to contamination.
Normally contamination means throwing the pads away. If you don’t have this option, you can either leave them as they are, with reduced braking capability, or you can try and clean them. Be aware that the cleaning process may not help, and may crack the pads, rendering them useless.
Disk brake pads are made from heat-resistant, high-friction material. They also tend to absorb oil well, so it leeches into the pads rather than just sitting on the surface.
This means that wiping them with isopropyl alcohol isn’t going to clean them off in the same way as it cleans disk brake rotors. Instead, you need to draw the contaminants out from the pads.
DOT4 boils at around 446°F (230°C), mineral oil boils at 500 – 626°F (260 – 330°C). To remove these contaminants successfully, you either have to wick them out of the pads at or above their boiling point, or clean them out with a surfactant detergent which breaks them down and carries them away.
Cleaning the pads with fire
The old-fashioned way of doing this, for car brake pads, is to drench them in white petroleum gas, set them alight, then sandpaper the surfaces to burnish them after they have cooled down. This works by having the white gas permeate the pad and pull the contaminants out of the pad as it burns. Bike disk brake pads are much smaller and the compound may well crack or come away from the metal backer plate when heated in this way.
Cleaning the pads with water
A less spectacular although slightly safer alternative is to use a good detergent at a high temperature for a sustained period of time. Otherwise known as putting the pads in a dishwasher.
In my experience, dishwashers do sometimes clean pads sufficiently to re-use, but the pad will need subsequent burnishing with sandpaper. Make sure it’s someone else’s dishwasher, as the oily dirt on the pads can leave marks inside the machine.
Before reinstalling the pads
Remember to fix any leaks and clean off the disk and the inside of the caliper with isopropyl alcohol before re-inserting or replacing the pads. The same applies when you replace the dodgy pads with new ones.