There can be many symptoms of a bad inlet valve on your pump. The most common of which is difficulty priming your pump. Replace it with a new Graco quality inlet valve. Inlet valves are a wear items and replacement is recommended for regular pump maintenance. Don't overlook the outlet valve #289878 (except SR7 uses # 243094) as these are also wear items and can cause pressure and prime problems.
Always check your manual for your application!
Graco Magnum X5, & X7
Graco Magnum SR7
Graco Magnum LTS15
Graco Magnum LTS17