Where is the grease on my front brake caliper coming from?

Question

I've had a new road bike for three months to commute. I live on the east side of Hawai'i island so I'm often riding in the rain. A few weeks after I first bought the bike the front brakes started squealing, so I tried cleaning the brake disk with isopropyl alcohol. Unfortunately this didn't fix the problem. I took the bike to a mechanic and he found that the brake pads were contaminated and had to be replaced. This was three weeks ago. Now the squealing is happening again with the new pads, and I've noticed that my front brake isn't working as well as before when the pads were new. I checked out the brake caliper and the outside casing has a greasy layer on it, the sort that's sooty and black when you run your finger over it. No doubt there's also grease inside the brake caliper that has contaminated the pads again, and again I'll have to get new ones.

Does anyone have any idea where this greasy stuff is coming from? Or how I can prevent it from getting into my brakes? I don't know if it's coming from the rain, the road, car exhaust, the brake hydraulics themselves, or somewhere else. My rear brake hasn't had any problems since I first bought the bike 3 months ago, the grease only seems to affect the front. I've also been very careful not to touch the brake disks, so I'm really confused as to how they're becoming contaminated. Any information or advice is appreciated, as I'd really like not to have to keep shelling out for new brake pads every few weeks.

Answer 1

Assuming you have hydraulic disc brakes, the most obvious explanation is that you have a brake fluid leak in the front caliper. If the grease isn't appearing anywhere else it must be coming from the caliper itself. Shimano uses mineral oil as hydraulic fluid which will feel greasy, the black color is road dirt sticking to the oil.

You can probably prove this by thoroughly cleaning the rotor, pads and caliper (inside and out) with degreaser, reassembling and applying the brake hard several times. Remove wheel and pads and look for fluid in the caliper (the pads are already contaminated so this won't do any further harm). DON'T try this with the rotor and pads removed as you will over-extend the pistons and they may come out of the caliper body, resulting in all your fluid spilling on the floor. Alternatively you can use a bleed block in the caliper to retain the pistons.

If the bike was bought new the store you bought it from should fix or replace the caliper for free.