Brake fluid replacement Whilst replacing my pads I made the mistake of pulling the brake lever,  which pushed the piston too far, and thus lost loads of fluid. 

I've searched online for the type and quantity of fluid to replace it with, but I cannot find anything specific to my Specialized Hardrock. 


What kind of fluid do I need?
How much will I need for one brake system? 
I've included picture of top and bottom of system for conformation of fluid in and out? Fluid in, I think;
fluid out.
could someone point me in direction of a video on replacing the fluid on this specific bike?
You have Tektro HDC 300 hydraulic brakes.

Follow the bleed procedure as described in the proper Tektro manual; https://www.tektro.com/upload/Product/F_20180828170966wPjDlH.PDF

The bleed process also adds additional fluid to the system so that will replace what you lost.