His mayoralty coincided with the 1964 Zagreb flood, the deadliest and costliest natural disaster since the city's incorporation, and he oversaw the rebuilding of the affected areas, including the construction of 26,000 new flats and houses.

He was a close associate of the main political figures of the Croatian Spring – SKH reformist leaders Savka Dabčević-Kučar and Miko Tripalo.

[1] After the suppression of that movement in late 1971, tens of thousands were expelled from the SKH, including 741 high-ranking officials such as Pirker, Dabčević-Kučar and Tripalo.

[8]: 399  The long-term plan for new housing units eventually proved overly ambitious,[8]: 409  but the program did set the city's development strategy for the next three decades.

[4] Pirker's term yielded 26,000 new flats and single-family houses, including the realisation of several new superblock neighbourhoods in Novi Zagreb.

[3] In 1963, Pirker opened the Sljeme cable car, the longest single-cable lift in Europe at the time, whose construction began during Holjevac's mayoralty.

Along with Savka Dabčević-Kučar and Mika Tripalo, he took a leading role in the Croatian Spring, a reformist and decentralisation faction of the SKH seeking greater economic, political and cultural autonomy of SR Croatia within Yugoslavia with support of a wider grassroots movement.

[4] The Croatian Spring was repudiated by President Josip Broz Tito in December 1971, and Pirker and others were forced to resign their positions.

[14] Due to his role in the Croatian Spring he remained unpopular with the Yugoslav government, which allowed him to fade from public memory.

[20] In 2021, a year after the 2020 Zagreb earthquake and following the death of controversial mayor Milan Bandić, a biography of Pirker by Goran Beus Richembergh began trending on Facebook and in Croatian news media, which contrasted the achievements, including the recovery from the flood, of the poorly remembered Pirker with Bandić's "miserly and barren"[20] mayoral work.