From the Jeep factory service manual...
The ESM switch mentioned above is essentially a vacuum switch that closes at a very slight vacuum, as in a few inches of water. The purge solenoid uses the relatively high vacuum in the intake manifold at part throttle to pull a slight vacuum on the fuel tank and evap canister which closes the ESM switch, then the PCM closes the purge solenoid with an expectation that it will hold vacuum (ESM switch staying closed) for some time period -- If not, there's a large leak. Can't quite see how adding a pipe to the brake booster hose would affect how this works.