You ask we answer. Here's the latest in our "Ask A Mechanic" series.
Q: I can never get the front brake back to original power. I bleed them and it has power but not as much as it had off the show room floor.
A: There are a number of things that could be wrong. Not only could there be a slight leak at the bleeder as mentioned, there could be numerous other places that could be leaking, however a leak would leave evidence in time. Leaks can occur at the brake line fittings and banjo bolts, at the caliper piston, at the master cylinder piston but you would in time have some visual brake fluid. Even the slightest leak would collect dust near the leaking area.
What is most commonly over looked is the wear limit of the rotor. People change pads often but neglect to change the rotor, measuring the rotor with a micrometer or caliper set, and compare to wear limit specs. Most rotors will have this wear limit printed, etched, engraved, stamped somewhere on the rotor. See here for an example. Notice, in the bottom right corner on the rotor (Min Thick 4) this means that this rotor, per Galfer, has surpassed wear limits at 4mm or less. The measurement needs to be taken where the pads ride. A micrometer works best as a caliper will normally give a false reading because most rotors develop a ledge on the outer edge of the rotor.
Aside from the rotor being out of spec a rotor's surface starts to deteriorate. As the rotor wears over time the pads make the rotor slicker and slicker. Nothing rides like a new rotor that is broken-in. A rotor and pads that are NOT broken in will not stop. The first couple laps will be terrible on a new pad and or rotor until they "break the glaze" or heat up and break-in.
I like to use a DA Sander/Grinder with a mild scotch bright pad to scuff the rotor surface and the pad surface this helps remove any rust inhibiters on a new rotor. Rotors normally have a light coating of oil to keep them from rusting while on the shelf. Also, when they are cut a lot of the machine processes use a cutting/cooling oil that ends up on the product out the door. Same with the pads - just a quick scuff with the DA and a scotch bright pad breaks the bit of glaze that occurs when new. I will also scuff an old rotor every once and a while to help take that shine off. This dramatically improves an old rotor that still has some life left. This is something that mechanics have been doing on brakes for vehicles for years.
People also don't look closely at what they are installing, like new pads for instance. I have seen many cheaper brand pads that have so little meat on them, brand new out of the box, that they are almost worn out right away. I had a set the other day of cheap pads that I put on and they only had half the pad on them compared to a new set of premium pads. There is a spec for the pad thickness and it should be checked even on new pads if you suspect that they look a little thin. Grab the book and check them.
People also don't consider the pad compound used off the showroom floor vs. what they are installing aftermarket and sometimes end up installing a different compound that may not perform the same as the OEM pad. See Best Brake Pad Material for Dirt Bikes for more information.
Beyond that upgrading to an oversized front rotor (not-applicable to ATVs) can increase breaking power beyond what you had on the showroom floor and is a valuable option that might eliminate this very problem and even make it better than it was.
Other issues could also include deterioration of the brake fluid. Or incorrect brake fluid. The dot spec of brake fluid pertains to the heat it can withstand before it breaks down. Moto brakes get hot, are small and don't dissipate heat well. Lower spec and cheaper brake fluids break down fast, and all brake fluids will need to be replaced at some point. Degradation of your fluid can cause symptoms just like the ones described.
Worn out or malfunctioning master cylinders and calipers can also cause problems so it may be time to rebuild or replace a caliper or master cylinder.
OEM Brake lines have been known to wear out. Sounds funny but the OEM lines, depending on the style used, can stretch as the hydraulic pressure builds and the lines can actually budge when engaging the brakes. It's very slight and you can't really see it but you can feel it in the brakes. This is why most race teams do not run OEM lines. Aftermarket lines are normally steel-braided that helps put some reinforcement around the line giving it more structure and reducing that feeling drastically.
One last thing is that people often struggle with bleeding the brakes - most are just doing it wrong. Even some of the best mechanics can have a tough time without the right tools. A brake bleeder set is a great investment if you're doing your own maintenance on your brakes. I'm not so sure of the little one man bleeders like this one being useful at all, but this BikeMaster works great: or even better this Mityvac. One of these is going to make night and day of brake bleeding, and changing brake fluid, it's also handy for a bunch of other stuff.
For more information on brakes, check out:
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