AP Racing GP Master Cylinder
What can we say about a simple brake master cylinder? After all, your bike stops just fine, so why spend $500 on a little thingie with a lever and a bunch of nasty fluid in it? It's simple really: If you want to go fast on a race bike, you've got to do three things better than anyone else -- stop, turn and accelerate. But suspension setup is rather complicated, even for an experienced racer who has access to a shock dynomometer. Likewise, high-power motors are expensive to build and even costlier to maintain. That leaves braking, and in this area, lever feel and power is everything.
Of course you need high-quality brake pads and good rotors, but most OEM sportbikes made in the '90s already have those. That vague, spongy feel you get at your lever is caused by two things -- cheap rubber lines and low-quality stock master cylinders. So the first thing you do is go buy some "dash-two" steel-braided brake lines; they're made out of very small diameter teflon tubing and shrouded in a steel braid to prevent abrasion holes from rubbing against fenders, triple clamps and so forth as the wheel moves up and down.
Theoretically, the diameter of your brake line shouldn't matter -- fluids should be uncompressible at these pressures, so pushing X amount of fluid from a master cylinder should result in the same amount getting into your brake pistons' cylinders. Unfortunately, it's an imperfect world we live in. Brake lines, especially the large-diameter ones that come standard on new motorcycles -- and even the "dash-three" lines that are common in most aftermarket kits -- flex, in some instances quite a bit. Thus, since the dash-two lines are of a considerably smaller diameter, there's less cross-sectional area to flex. While the percentage of total area Vs. flexed-out expanded area might be the same ratio as larger lines, the resulting amount of fluid volume wasted is significantly less -- remember, we're pushing the same amount of fluid through the lines irregardless of the lines' diameter. The result from this little technical tirade? A much-improved lever feel that's both firmer and feels more "in touch" with what the brakes are doing.
With that solved, we turn to problems that revolve around master cylinders. First, air is lighter than brake fluid. Thus, it rises to the top, making it nearly impossible to get all of those tiny bubbles forced out of the brake piston's bleeder screws. AP Racing's GP Master Cylinder has a bleeder screw adjacent to the fluid in line connected to a remote reservoir, an especially handy design that enables you to easily bleed the master cylinder itself. Also, you bleed your brakes at the top instead of crouching down by the wheel, which results in less air in the system.
Despite the AP's high price, a fluid reservoir isn't included, so we fitted one from Yamaha's YZF600. Remote reservoirs are effective because you don't have to worry about odd handlebar angles that might uncover the piston on an integrated reservoir/master cylinder, causing the piston to suck air, or seep fluid out of the reservoir's vent hole. Be warned that you can't put any old fuel or oil lines between the AP and a remote reservoir because they'll "weep" water into the system, a really bad thing for brakes.
The AP's real advantage is the master cylinder itself. Made to exacting tolerances, the unit is second to none in performance. But the really trick part is the adjustability built in: its master cylinder ratio is variable from 16.25mm to 21.9mm. The lower the bore size of a master cylinder, the less fluid you push per X amount of lever travel. Thus, you get more leverage on the brakes, making the smallest setting perfect for single six-piston calipers or small dual-four setups, while the 21.9mm setting can push a set of the largest six-piston calipers made. Adjusting the ratio is easy -- merely release a set screw with a 3mm allen wrench and turn a knob located at the lever's base. Naturally, the lever's position relative to your handlebars is fully adjustable by loosening another set screw and turning a threaded adjuster.
The piston bore size isn't actually adjustable -- the leverage ratio between the brake lever and the piston is. If you look at a standard master cylinder, the lever rotates around a fixed pin about a half inch away from a tab that pushes the main piston in. AP turned their piston design around so it's a pull type, and made the lever's pivot adjustable, so you can move it closer or farther away from the piston itself. The lever doesn't move, just the pivot point. If you move the pivot point away from the piston and towards the handlebar end, lever travel is reduced for a given volume of fluid. Thus, you've reduced the amount of leverage and increased the amount of fluid pushed down the lines, effectively increasing the bore size. Conversely, moving the pivot as close to the master cylinder's piston as possible increases your leverage on the bar and effectively reduces piston size.
The AP's only downside is it's astronomical price tag, which is compounded by the cost of replacement levers -- they're about a hundred bucks each. This poses a difficult question: Is this necessary for street applications, or even for racers? For the former group, probably not, unless you're building a custom sportbike or just have a lot of money to burn and want the best. But for racers? In our experience, AP's GP Master Cylinder is the easiest and most lasting performance advantage you can buy. Unless you wad your bike up into a little ball and totally destroy the AP, it's a one-time investment that'll give you supreme confidence under braking. Of course the caliper is fully rebuildable, so it should last for many seasons, and could be the difference between winning and losing.
AP's GP Master Cylinder would have netted our coveted five star rating were it not for its lofty purchase price.