Two prototypes were completed in 1953, one with Duple coach bodywork and one with Park Royal bus bodywork. Production vehicles entered service from 1954. The last Reliance entered service in 1981.
Following successive changes to Construction & Use regulations, the maximum length of the Reliance was increased twice from the original 30': firstly, to permit an overall length of 36' from 1962; and later, to permit a length of 12 metres.
Various AEC engines were fitted during the chassis's production, including the 7.7-litre AH470, 8.1-litre AH505, 9.6-litre AH590, 11.3-litre AH690 and 12.4-litre AH760. Transmissions fitted to the Reliance include an AEC synchromesh gearbox, AEC Monocontrol semi-automatic epicyclic transmission, and ZF synchromesh gearbox.
The Reliance had the Leyland Tiger Cub and, from 1959, the Leyland Leopard as its major competitors throughout its life, even though they were built under the same ownership from 1962 onwards; later, the Volvo B58 won a number of customers' orders away from both the Reliance and the Leopard.