If you can use a saw and a drill then you can make this mud guard very easily. It takes only a few minutes to create something that looks professional and works well.
The brakes on my bike are Shimano SLX which I have added retainer bolts instead of cotter pins to keep the pads in place. you should be able to do this modification on any brake calliper which uses a central screw in retaining bolt to keep the pads in.
You can see the problem in the pictures - in one the calliper is clean and the other is dirty after a ride through miles of mud. The mud reduces the braking power by contaminating the pads and acting as a lubricant. That is also when you get squealing brakes.
The brake callipers and pads have an opening to them, for increased ventilation. In the wet , muddy cold winter conditions ventilation is not a problem but keeping the brake pads dry and clean is.
The mudguard woks by stopping rain and mud getting into the gap and therefore keeps the pads clean and dry.
You will need;
1. Two picture rail hooks (the type that is in the photo below which hang on a rail rather than get nailed into the wall)
2. Rotary tool with cutting disk (or hack saw)
3. File or sanding attachment for rotary tool
4. Drill with an assortment of metal drill bits
5. Safety gloves
6. Safety goggles
7. Screw in retaining bolt if you are converting from a cotter pin set up.
Before we get into the details of how to make this Instructable I have decided that this modification needs a name. "Brake calliper mud guard" is too boring. I am going to call it a "Picture Hook Anti Sludge Thingie" (PHAST). Clean dry brakes should give you greater confidence and make you faster anyway so this sounds appropriate. Anybody else got any better ideas?
LEGAL DISCLAIMER (well, I am a lawyer after all!): If you decide to do something similar to this to your bike you take full responsibility for any damage to yourself, the bike, any other person or any other property as a result of such a modification. Brakes are an essential safety item on your bike and you should not touch them or modify them unless you know what you are doing. Do not attempt this modification if you are not confident of your own mechanical skills. If in doubt speak to your local bike shop mechanic (who you should be supporting anyway).