  Minimizing leakage from computational states is a challenge when using
many-level systems like superconducting quantum circuits as qubits. We realize
and extend the quantum-hardware-efficient, all-microwave leakage reduction unit
(LRU) for transmons in a circuit QED architecture proposed by Battistel et al.
This LRU effectively reduces leakage in the second- and third-excited transmon
states with up to $99\% $ efficacy in $220~\mathrm{ns}$, with minimum impact on
the qubit subspace. As a first application in the context of quantum error
correction, we demonstrate the ability of multiple simultaneous LRUs to reduce
the error detection rate and to suppress leakage buildup within $1\%$ in data
and ancilla qubits over 50 cycles of a weight-2 parity measurement.
