I am having to add brake fluid way more than I should be on my 1994 F150. There are no leaks around the master cylinder, or on any of the fittings that are visible under the hood. Where should I start looking for the leak.... What are the usual places that start to leak on this truck.
Why not trace all the lines from the master cylinder to each wheel. if it is a line it is most likely be coming from the rubber coated flex line near the wheels.
The calipers on the front...check for any oily stuff coming out of the caliper piston, unfortunately, you may have to remove the brake pads and caliper to get a good look. If it is leaking bad you might notice it if you take a good long look while under the front end.
If the rear is leaking, you proabably need to pull the wheel and drum off the truck. The brake cylinders are mounted inside the drum and can leak out either end.
Loss of any brake fluid at all is unacceptable, whether it is leaking out or boiling out, I honestly can't say which is scarier. It is something that needs attention ASAP.
PS My 92 brakes are still original stock, except the rear brake cylinders, they leaked and have been replaced with new ones, they are 10 US dollars each
Are you still able to use your brakes? If there is a leak and no air is being introduced into the system one of the most common places is the rear wheel cylinders. Look under your truck at the inside of the back tires, if they appear wet, than you need to invest in some wheel cylinders and brake shoes.
I would definitely check it out ASAP. Like everyone else says, you need a thorough inspection done to find the leak. If you can't find it anywhere, the master cylinder can leak into the brake booster. If that's happening, you need to replace the master cylinder and the booster.