Indeed the mystery is very deep. But using racing brake fluid with 594 F boiling point didn't change anything, at that temperature all plastic components and cables around should have melted but are ok. My bike consumes some fluid and I purge the air on a regular basis. The nipple is a one-way spring loaded gasket-sealed unit.
Thanks for contributing to the conversation. Your bike sounds normal. As the pads and rotors wear the system consumes some brake fluid. If you are still having to purge regularly it sounds like you aren't getting it all out to me. Good enough for a while until the compression ratio of gas to liquid takes more than the master cylinder can compensate, pump it again. Bleed it again. Harder, longer....
Sounds like you have Speed Nipples installed with the springs to prevent air during bleeding. They still have to be torqued on the metal to metal bevel that seals off. The springs are just used during bleeding. Tighten them back up.
An interesting read on brake lines for us in the US.
Brake & Front End serves repair shops conducting a high volume of undercar repairs by providing application-specific technical information and solutions to address emerging trends in the undercar repair segment.
Brake hoses are the most government-regulated components on a vehicle. Anyone making brake assemblies must be registered with the Department of Transportation (DOT). All aftermarket hose, fittings and complete hoses must conform to FMVSS 106 and SAE J1401. These tests are demanding and often exceed what a vehicle will see in the real world.
The DOT standard in a nutshell says brake hoses must be flexible in a wide range of temperatures while having a predictable expansion rate so the pedal feel and ABS response is the same in winter and summer. It also specifies that hoses must be able to bend and twist at certain angles without collapsing, kinking or bursting.
FMVSS 106 and SAE J1401 do not specify construction or materials. These documents outline a test procedure that completed hose must pass.
These tests and standards include:
• Markings: Each hydraulic brake hose, except the original hoses, have at least two clearly identifiable stripes of at least 1/16th of an inch in width, placed on opposite sides of the brake hose parallel to its longitudinal axis. These are called “torque stripes.” One stripe may be interrupted by the information printed on the hose. These are to prevent twisting during assembly and installation.
• Burst Pressures: The hydraulic brake hose assembly must withstand water pressure of 4,000 psi for two minutes without rupture. Hose that is 1/8 inch, 3 mm, or smaller in diameter must be tested at 7,000 psi.
• Whip Testing: Brake hoses are continuously bent on a flexing machine for 35 hours at pressure.
• Tensile Strength: A hydraulic brake hose assembly must withstand a pull of 325 pounds without separation of the hose from its end fittings during a slow pull test, and shall withstand a pull of 370 pounds without separation of the hose from its end fittings during a fast pull test.
• Cold Resistance: A brake hose is chilled to a temperature below minus 49º F for 70 hours shall not show cracks visible without magnification when bent around a cylinder.
• Chemical Resistance: In order to test the hose’s chemical resistance to brake fluid, the hose is subjected to a temperature of 248 degrees Fahrenheit for 70 hours while filled with SAE “Compatibility Fluid.” After this the hose is subjected to a 4,000 psi burst test.
• Ozone Resistance: A brake hose assembly is exposed to ozone for 70 hours at 104° F. Any cracks seen at a specific magnification is ground for failure.
• Fitting Corrosion Resistance: After 24 hours of exposure to salt spray, a hydraulic brake hose end fitting must show no base metal corrosion on the end fitting surface except where crimping or the application of labeling information has caused displacement of the protective coating. What FMVSS 106 does not test for is damage that can take place over a longer period than 35 or 70 hours. It also does not take into account “acts of God” like impacts with road debris and installation errors.
There is not a recommended replacement interval for brake hoses. Brake hose conditions differ from vehicle, drivers and the environment. Inspection is the only way to spot a problem before a brake failure.
Brake hoses should be inspected visually and with your hands. It might be helpful to have someone pump the brake pedal to spot a defective brake hose. Failure typically occurs at the ends of the hose. This is where a hose flexes due to suspension/steering movement. It is also exposed to damage from debris and heat from the brakes.