I have a 2012 evo and looking at the sight glass for rear brake fluid it looks low I took off cap to fill but the level hasn't gone up and fluid was at the top of the tube should I fill with rubber insert fitted or removed and why has level not risen in sight glass 

Thanks 

Rubber insert is a diaphragm which allows fluid to move. There is no atmospheric breather hole, if there was you would get fluid leaking out of it, due to the nature of two wheeled bikes. It’s the same for the front brake fluid reservoir. This is how it is on all hydraulic brake systems for bikes. The sight glass is there to show you have fluid in the reservoir, it’s not a level and when you put the rubber diaphragm back it will displace some fluid.