The best way to find out where the leak is coming from is to inspect/observe, if possible. As far as what parts could be the culprit, there's the brake master cylinder, a hydraulic brake hose, or a brake caliper.
You put the brake fluid inside the brake master cylinder
D O T ( 3 ) brake fluid
The valve is located by behind the intake manifold. It has 6 brake lines coming off of it. Two Lines for the Brake Fluid Reservoir, Two on for the back tires and Two for the Front Tires. If you have brake fluid leakage at the firewall, replace the pistons in the proportioning valve with good ones. It seems that lots out there may leak, some don't.
The brake fluid on a 2003 Avalanche goes into the fluid reservoir. The reservoir should be bolted onto the drivers side of the firewall.
The kind in a can.
Use DOT3 brake fluid. It is written on the top of the fill cap.
Dot 3
According to the 2003 Ford Mustang Owner Guide : ( DOT 3 brake fluid meeting Ford specification ESA-M6C25-A )
According to the 2003 Ford F-150 Owner Guide : The clutch fluid ( and brake fluid ) is DOT 3 brake fluid meeting Ford specification ESA-M6C25-A
it says on the master cylinder cap. it should be Dot 3 brake fluid
No.No.
According to the 2003 Ford Explorer owners manual : ( DOT 3 brake fluid , meeting Ford specification ESA-M6C25-A )