Hydraulic Circuit Modifications

[Shazaam]

Questions regarding the size and location of the hydraulic fluid reservoirs come up from time-to-time, so lets take a moment to address some safety concerns regarding modifications to the clutch and brake actuation circuits.

To some owners, the hardware and its placement is considered to be quite unsightly so they have taken various approaches to replacing the mounting brackets and tanks with more attractive items and relocating them to less obtrusive locations on the bike. The Rizoma billet reservoir tanks are a popular replacement item but introduce another concern, as you will later see.

The first question that comes up is whether a smaller-than-stock replacement reservoir can be used given that the size of the stock reservoir design is based on the need to always maintain an adequate fluid level.

Clutch Reservoir. The stock clutch reservoir is designed hold a volume sufficient to replace the fluid displaced by the slave cylinder when the clutch plates wear over their service lifetime. However, clutch plate wear is a slow process so a smaller reservoir, refilled more often, would also seem to work. Also note that the volume of clutch fluid displaced over time is quite small yet the stock clutch reservoir is relatively quite large.

Brake Reservoir. The brake reservoir is designed to be larger because the volume of fluid displaced by the caliper pistons is larger than the clutch slave piston. But again, a smaller reservoir (with the fluid checked and replenished more often) would work just as well.

So how much reservoir volume do you actually need to maintain brake and clutch function in the short term? Maybe a thimble-full. As some have pointed-out, a short section of clear tubing attached to the master cylinder holds enough fluid to accommodate normal wear experienced during a day at the track.

The second question to consider is whether the reservoir can and should be moved off the handlebars to a more remote location location. In particular, should the reservoir be mounted below the master cylinder.

Since the master cylinder will work properly with a very small reservoir attached, if you elevate the reservoir above the master cylinder and properly fill it, assuring that the tank line is free of air bubbles, and verify that the tank diaphragm and cover are air- and leak-tight, you can then mount the reservoir tank below the master cylinder and the master cylinder will function correctly, although I don’t advise it.

That said, my recommendation is, for safety reasons, to leave the stock design and components in place. My reason?

LEAKS.

Keep in mind that motorcycle master cylinders, unlike automobiles, have no redundancy. That is, there is no separate second piston and reservoir, so if you loose fluid you loose braking.

The reason that the stock reservoirs are clear plastic and placed up near the instrument gauges (ugly, up and in your face) is that they allow you to periodically scan for rapid changes in fluid level and consequently to avoid impending catastrophic brake loss while you’re riding. Checking reservoir fluid level is part of your pre-flight checklist so if you’ve tucked them away or replaced them with billet units that don’t’ allow you to see level changes at a glance you’re depriving yourself of important safety information. Out of sight, out of mind.

Further, if you’ve mounted the reservoirs below the master cylinder, you loose the time safety cushion before failure that an over-capacity gravity-fed source of reserve fluid provides. If a seal or fitting starts to leak, any reservoir that’s mounted below the master cylinder cannot keep the master cylinder full of fluid, so at some point you will loose brake and clutch functionality.

Automobiles use a low brake fluid level dashboard indicator light to give an alert, you need to use the computer between your ears.

[bananaboat]

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