I have a 8.1 chevy engine in a 05 bounder. I was looking under the hood today and noticed a plastic part that has broke off from the front of the master cylinder.It is between the two metal brake line...
The brake pressure differential switch has a blue wire extending from it to the nearby harness. If it is broken you will lose one of your brake problem indications and may lose certain ABS functions.
You should get it replaced for safety sake.... :)
The one on the side of the reservoir is the level indicator for the brake fluid.