A breath actuated metered dose inhaler may include a canister fire system configured to fire a medicament containing canister in response to patient inhalation. The canister fire system may include a pneumatic force holding unit and having a rest configuration in which a metering valve of the canister is in a refill configuration; a prepared configuration in which a canister actuation force is retained by the pneumatic force holding unit and the canister fire system is actuatable by patient inhalation induced airflow; and a fire configuration in which the metering valve is in a dose delivery position. When in the prepared configuration, the force retained by the so pneumatic force holding unit may be reduced by less than about 6% over a period of 5 minutes, preferably less than about 3% over a period of 5 minutes.