Jimm 4/21/2013This may indicate a faulty power brake diaphragm (power brake booster). To check the booster, use a handheld vacuum tool to apply a vacuum to the booster via the vacuum hose and fitting on the side of the booster housing.
The power brake booster should hold the vacuum applied without leak of loss of vacuum over time. If not, replace the brake booster unit. For the replacement parts, try the many on-line parts sources, such as; www.autopartswarehouse.com or www.rockauto.com - to list only two. For example, they (RockAuto) list the power brake booster from around $126.00 (minus core charge) for your vehicle.