This makes it very difficult to bleed the brakes but worst of all it is down right dangerous!
I was recently tasked with reconditioning a Honda CB-500 Four that had been left in the weather for a few years. When I saw the bike one of the first things that I noticed was the “front brake reservoir delete” or in simple terms a piece of Tygon tubing with a plug in it. I wasn’t impressed but had seen something similar plenty of times on the motorbike blogs and on race bikes.
Then when I went to bleed the front brake I had one squeeze of the lever (with the bleed nipple open) and then nothing. No brakes. No pumping. Nothing and the tube was still full of fluid.
A brake reservoir needs a diaphragm in the top for two purposes. The first is to seal out moisture. Brake fluid is hygroscopic*, when it absorbs moisture it’s boiling point drops and so does braking performance. The second purpose is to flex and allow the fluid to be drawn into the master cylinder to top up the volume when there are any leaks or wear of the brake pad material.
In Australia the Australian Design Rules (ADR) number 33 covers brake systems for motorcycles and mopeds – https://www.legislation.gov.au/Series/F2007L02707
The section covering the brake reservoir states;
1.5.2 The capacity of each sub-system reservoir compartment must be not less than one and one half times the fluid displacement resulting when all the wheel cylinders or calliper pistons serviced by the reservoir move from a new lining, fully retracted position, to a fully-worn, fully-applied position. For the purposes of this clause, “fully-worn, fully-applied” means that the lining is worn to whichever of the following conditions allows the greatest shoe or pad movement:
So what does that mean? It means there should be more than enough fluid to go through normal use with full wear of the pad material or even allow a few applications with a small leak. To calculate that volume take the diameter of the brake caliper piston, 3.8cm in this case, divide by two to get the radius = 1.9cm. Use the formula πr2 to calculate the piston area and times that by the thickness of the pad material (in the case of the CB-500 Four with a single piston floating caliper it is both pads) = 1.0cm. So the volume is;
Volume = πr2 x thickness
= π x 1.92 x 1.0
What was the volume of the tubing with a ~0.7cm internal diameter and 7cm long?;
Volume = πr2 x thickness
= π x 0.352 x 7.0
Well short of the 17cc the ADR specifies.
It can be a different case for Moto-GP bikes which are trying to save as much weight as possible and have the brake fluid replaced at every race. Nor do they have to comply with the ADRs but if they fail they are less likely to put lives in danger (crowds are behind safety barriers).
Don’t be sucked into making stupid modifications just for looks. There are plenty of options out there that are not unsightly.
* Brake fluid is hygroscopic. All except DOT-5 which is a silicone based fluid.