Out of necessity I have gotten much deeper into the ALDA than I wanted to. The unit has two sealed brass barometric cartridge's which sense baro pressure and push on each other to set the initial position of the enrichment rod. The enrichment rod, the one that goes into the IP is sealed by a cup seal on the shaft.( this is the one that usually leaks) The upper cartridge is attached to the case and the bottom one is free to move. When pressure from the intake (turbo boost) is introduced to the case the bottom cartridge is pushed out causing the IP to flow more fuel. The easy way to test a ALDA is to hook the line going into the alda to a mity -vac (use the pressure output ) and see if the rod extends as pressure is applied. The unit is not designed to hold vacuum, only pressure. Also the port does not apply this pressure to the inside of the baro cartridge and therefore will not damage the unit. ( dont apply more than 10 psi)