Ford E series AC vent vacuum fix
Many of the Ford vans have a problem with the ac vents not working or defaulting to the defrost vents when on ac or heat. Mine is a 1999 E450 but the E350 as well as the 450's used for C class motor homes and the F150's all have the same issue. The vacuum accumulator and check valve are located between the fender well plastic on the right hand side and the battery in a space inaccessible without removing the fender and the plastic behind it. The common fix is to run the red vacuum line off the tee on the rear of the motor that comes across the right side right above the battery directly to the ac vacuum line at the right side under the dash in front of the passenger seat, this bypasses the bad accumulator, check valve or cracked line inside the fender you can't get to. When you bypass all that the vents all work till you try to pass a car of go up a hill and the engine vacuum drops, the dash vents close till you let off the gas and the engine vacuum builds back up.
a check valve, a sealed tank of some sort and a T to fit the small lines are all the parts required to fix it. I used a 134A can drilled on the top and tapped to 1/8th npt to fit a 1/8th npt x 3/16 fitting screwed and lock-tighted into the top.
The check valve goes to the red vacuum line over the battery, the Tee goes to the home made vacuum accumulator, the other side of the check valve and the line to the dash. The check valve lets the accumulator hold vacuum when the engine vacuum falls. The tank holds it's vacuum and the vent doors in place till the engine regains vacuum.