Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 26; Flight Crew Type 2357
Communication Breakdown; Training / Qualification
Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown
General Maintenance Action
Human Factors; Environment - Non Weather Related
This was the second leg of the day. I was the First Officer on my last day of a three-day reserve pairing and was assigned a ZZZ turn from ZZZ1. I have flown with this Captain before. During the initial briefing we briefed that a lack of proficiency due to reduced hours was an additive condition and that we needed to be extra vigilant to keep each other out of trouble.  In my case; it had been 40 calendar days since my last flight. We were preparing to push back from the gate in ZZZ. We were on time. I called Ground for clearance to pushback; received it and notified the Captain.  He gave the standard talk to the ground crew and I was clearing the right side of the aircraft for pushback.  I did not noticed that the parking brake was still set.  Ground Crew commenced pushback and the shear pin in the tow bar broke.More attention to switch position; avoid expectation bias and more flying wouldn't hurt.
We were not rushed and there were not additional threats. I just stated to the push crew that the brakes were released and cleared to push; but I must not have pushed hard enough on the brake pedals. I should have verified the red Brake Released light was out. Since the brakes were still set; the tow bar pin broke and we swapped for a different tow bar. I just didn't verify as I should have.  We are flying less; so we need to be even more attentive.  Better attention to detail.
Air carrier flight crew reported the parking brake was still set during pushback resulting in a broken tow bar shear pin. Both pilots referenced lack of flying as a contributing factor.