Most cars built in the last fifty years have a power brake booster that uses engine vacuum to assist the driver with pushing the brake pedal. The vacuum usually comes from the intake manifold.
A power brake booster check valve ensures that air only flows in one direction. Air is sucked out of the brake booster. If the check valve leaks, then gasoline vapors from the intake manifold can enter the brake booster when vacuum drops during acceleration. Inside the brake booster is a large rubber diaphragm that can be damaged by gasoline. It is important to install a new check valve when installing a new brake booster.
With the most common brake booster designs, the vacuum hose runs to the check valve and the check valve fits into a grommet on the surface of the brake booster. The problem is that it is easy to assume the check valve is nothing more than a plumbing fitting that is not worth replacing.
New brake boosters almost always include new check valves. The old check valve might still get reused because the exterior of the old valve likely looks OK, and it may seem easier to shove the old valve into the new brake booster's grommet than it is to pry a petrified vacuum hose off the old valve's nipple.
Check valve (circled) next to master cylinder on power brake booster
On some brake booster designs the check valve is internal, and the vacuum hose connects to a permanently mounted fitting. On some big trucks, the check valve might be spliced into the vacuum hose some distance from the brake booster. A good, safe rule of thumb is whenever replacing a power brake booster, always use every new part that came in the box, even if it looks like nothing more than a simple plastic elbow or hose splice. That plastic piece might be as important as the new brake booster.
Tom Taylor,
RockAuto.com
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