GMC 2003 C4C042 Owner's Manual page 296

Checking Brake Fluid (Hydro Max System)
Apply the brake pedal several times with the ignition
off. Then clean one of the reservoir caps and the area
around the cap, and remove it. The fluid level should be
even with the bottom ring of the filler opening.
If it's low, add enough fluid to fill the reservoir to the
proper level.
6-42
The Hydro Max
brake fluid reservoir
is located in the
engine compartment
on the driver's side
of the vehicle.
Checking Brake Fluid (Hydro Boost System)
Apply the brake pedal several times with the ignition
off. Clean the cover and the side of the reservoir.
Check to see if the fluid level inside the reservoir falls
within the MIN/MAX level indicator on the side of
the reservoir.
If it's low, add enough fluid to fill the reservoir to the
proper level.
The Hydro Boost
brake fluid reservoir
is located in the
engine compartment
on the driver's side
of the vehicle.