92 caravan vacume problem

[frotrk]

Changed spark plugs in 92 grand caravan 3.3 and now under any load, light or heavy acceleration the air vents switch to defrost(default). Under steady acceleration first vents switch then brake pedel gets hard so I am losing vacume to booster. have checked all vacume lines, did not knock any off, and can not hear any leaks. All I did is change plugs. Any help out there
Clark