I agree with Joe & Mike, leave the cap alone but install a tank vent with high siphon/loop to start. OEM vapor systems have the tank vent line running to a vapor/liquid separator that drains back to the tank, the line to the canister, and then the control valve which the ECM drives. I think that the canister is being purged anytime the engine runs, to suck absorbed fumes out of the charcoal. Venting it only when on the brake doesn't seem like a good idea.