Rear brake master cylinder confusion on my 2010 ZX6R

[Hip Man]

While bleeding the brakes on my 2010 ZX6R (40K miles, recently picked up), I noticed the rear master cylinder sight glass shows low fluid, even though I filled it nearly to the top. I stopped bleeding to avoid pulling in air. Can anyone explain how the rear master cylinder works on this bike? Why isn't the level showing properly in the window?

 

[ClipOnBandit]

The rear master cylinder sends fluid from the reservoir to the caliper when you press the brake pedal, but they work separatley. Make sure to use fresh, sealed brake fluid since it takes in moisture quickly.The rear lines can have air stuck in some spots, so be patient while bleeding them.The piston only moves a little bit each time you press it, and it only goes one way.Hope this info helps

 

[Alma]

Sight glass can be misleading if the bike isn't perfectly level or if there's air in the system, also grime inside the window can distort the view

 

[Hip Man]

The rear master cylinder sends fluid from the reservoir to the caliper when you press the brake pedal, but they work separatley. Make sure to use fresh, sealed brake fluid since it takes in moisture quickly.The rear lines can have air stuck in some spots, so be patient while bleeding them.The piston only moves a little bit each time you press it, and it only goes one way.Hope this info helps

Thanks, that helps a lot, I'll double-check for air and keep bleeding slowly. I did use fresh DOT 4 from a sealed bottle, so I'll stay at it and see

 

[Hip Man]

Sight glass can be misleading if the bike isn't perfectly level or if there's air in the system, also grime inside the window can distort the view

Good point, yeah the bike might've been a little off-level when I checked, I'll clean the glass too and take another look after bleeding more