[Tsundoku]

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.

 

[K1W1]

Yes that is the ABS activating on the rear brake. Every vehicle equipped with ABS will do exactly the same thing if you brake hard enough.
The front and rear brakes have their own fluid reservoirs, The rear is near your right ankle when you are sitting on the bike. Adjusting the bars will not affect the rear brake and unless fluid leaked out of the handlebar reservoir then no air got in so there will be no affect on the front brake either.

 

[Tsundoku]

Great, thanks. I had a feeling it was the ABS. My last bike had a drum brake on the back, so it was basically use at all, so this is all new to me!

 

[K1W1]

If you have never owned an ABS equipped bike you should go somewhere and practice using the ABS so it does not scare you when it activates. Find an empty car park with a solid surface (pavement not gravel) or some place similar and get up to speed (say 30mph), make sure there is nobody behind you and grab the brakes as hard as you can and hold them as hard as you can until you come to a stop, Try it a few times and then when you are comfortable as the brakes activate try a small swerve left and right to simulate avoiding whatever caused you to brake in the first place.
The problem with ABS is that many riders panic when they feel the ABS operating in an emergency and let go of the brakes. You need to learn how it feels and to trust it and it may save your life one day.