On my 2003 4x4 Xterra, I had Replaced my Power Brake Booster & Replaced the Old Hose (Booster to Intake Manifold) and tested the check valve (air flow towards the Intake) and check valve is working properly, and inserted the valve into the new hose and connected....the problem I am having is I am still having a hard brake feeling (not much) and engine sputters every now and then.....Does anybody know what else it could be that is causing this ?