[augiedoggie]

Check caliper mountings and make sure calipers are not moving excessively when lever is actuated. Loose caliper or missing hardware or missing brake pad(it can happen) can require more hydraulic travel than your master can generate and allow excessive lever travel. Smaller bikes can be tested for master cylinder internal leakage by removing single caliper and clamping brake pad or flat metal plate to retain caliper piston(s). Once piston travel is prevented the lever is held tight for a time. If lever sinks slowly and no external leaks are apparent, you have internal master cylinder leakage and will require rebuild or replacement. Multi piston opposed twin calipers could make this method unworkable. You can be fairly certain if your calipers are not moving excessively and lever sinks with no external leakage you can probably safely expect your master has failed and leaks internally. Your ABS would likely not cause internal pressure drop.

 

[augiedoggie]

Never a good idea to revive a dead thread. Best to start your own thread with all relevant details for best success. Your solution is to reverse bleed air out of master by compressing caliper pistons a bit to force air back out. This will work temporarily. Rebuilding master cylinder can help for awhile but new replacement master cylinder is the permanent fix

 

[augiedoggie]

I have a theory. I suspect warm brake fluid cools and shrinks fluid displacement and draws air into master cylinder as fluid cools. Just a theory

 

[augiedoggie]

More than likely loose caliper mounting or possible brake hose failure. Hope you checked master cylinder lever free play. Too tight or zero clearance will also cause your symptoms.

 

[augiedoggie]

Be sure to adjust master cylinder lever free play exactly as specified in your service manual. This setting is critical to brake operation. Your master cylinder converts your lever pressure to hydraulic pressure and it's transmitted to calipers where it's hydraulic pressure is converted to mechanical pressure again. Your caliper mountings or rotor mountings or anything mechanical can cause excessive travel/movement to dissipate more hydraulic pressure/stroke. Imagine your master having the ability to generate 1 inch of movement to fully lock calipers normally but wear or damage has now required 2 inches of movement. Your lever needs to travel further. Loose wheel bearings, loose caliper mountings can all cause this additional travel. Lift front wheel off ground. Watch caliper movement when applying brake and rotating front wheel. Observe any excessive movement or travel.