I have a 98 dodge ram 1500 with the 5.9 in it and ive got a vaccum leak somewhere the front driveshaft will spin but no vaccum is going to the CAD to engage the front axle also i can hear a hissing noise at idle that will get slightly louder coming from the top of the motot but i cant seem to pinpoint its exact location any help would be appreciated

 

Toward the rear of the motor, passenger side. That's where the bag o' snakes is. Peak around back there, and follow the line toward the back, and down to the t-case. They are the solid plastic lines, that like to get brittle, and break. Chances are good, one of your has done exactly that.

 

Check the metal lines that run along the frame also, If you have a leak/bad seal at the cad it will allow in moisture and rust the inside of the line. Mine was so rusted it blocked all vacuum to the cad. I love living in the rust belt. NOT!