I noticed on my ride just now that the rear brake when pushed really hard shudders/judders, almost like it's locking up and releasing over and over. I hadn't noticed this before, has anyone experienced the same? I'm thinking it could be the ABS. Also, when I was adjusting the handlebars earlier they very briefly slipped and the brake fluid reservoir turned upside down for a few seconds. Could this have allowed air to get into the brakes and cause this issue? I'm pretty much a layman on the mechanical side of bikes so any help is appreciated.