Nothing in the vacuum booster or hose will get into the fluid or contaminate it. Something else had to be poured right into the master cylinder reservoir. That can include power steering fluid, engine oil, or transmission fluid. The fluid can also be contaminated by repacking wheel bearings during a brake job, wiping your hands on a rag, then using your fingers to reseat that bladder seal. The residue of axle grease on your fingertips is more than enough to contaminate the fluid. Wiping the oil from a funnel used to fill engine oil, then using it for brake fluid is also unacceptable. There is still too much residue left behind.
The only acceptable fix is to replace every part that has rubber parts or seals that contact the fluid. That includes the master cylinder, front calipers, rear wheel cylinders, three rubber flex hoses, combination valve under the master cylinder, height-sensing rear proportioning valve if used, (most trucks and minivans have them), and the anti-lock hydraulic controller, pump, accumulator, and hoses, if your truck has anti-lock brakes. You don't need to replace the power booster or vacuum hose. All of the steel lines must be flushed and dried before the new rubber parts are installed.
It is also important to understand that some people run into trouble when they just replace a few parts first, such as the master cylinder. That may appear to solve the problem for a while, but the contamination will leach back out of the parts that weren't replaced and recontaminate the fluid. The most overlooked part is the combination valve. It has rubber o-rings and seals. You can find it by following the two steel lines coming out of the master cylinder. The brass junction block they go into is the combination valve. The proportioning valve is built into the combination valve on cars. Its job is to limit fluid pressure to the rear brakes to reduce rear-wheel lockup under hard braking. Instead, most trucks have a height-sensing valve connected between the frame and rear axle housing. That's done because there can be such a great variation in weight distribution between empty and a full load. When the rear squats from a heavy load, more fluid pressure goes to the rear brakes. That valve assembly also has rubber o-rings so it must be replaced too.
Friday, September 16th, 2011 AT 11:02 PM