It faced serious opposition from riparian landowners and there were 16 petitions against it, but several influential civil engineers was called to give evidence, notably Bateman, Thomas Hawksley James Mansergh, and George H. Hill.

[7] During 1902, the Corporation looked at increasing its capacity, so that Cantref and Beacons could be used exclusively for water supply, and all of the compensation flow for the Taf Fawr, which would then be 7.75 million imperial gallons (35.2 Ml) per day, could be provided by the new reservoir.

[8] When the corporation decided to proceed, they elected for a reservoir that was considerably bigger than their 1902 plans, and in 1909, a new Act was obtained that doubled the permitted size to 1,260 million imperial gallons (5,700 Ml).

[9] Once Beacons was completed, the upper sections of the railway were lifted, and it terminated near a small bridge that carried the road to Penderyn over the Taf Fawr, above the location for the new dam.

In November 1910, a contract worth £201,077 was awarded to the contractor Louis P Nott of Llanelly and Bristol, which gave him six years from January 1911 to complete the works.

The railway from Cefn-coed-y-cymmer had been disused for twelve years, and his first job was to put it back into order, including reconstructing the viaduct that carried it over the Taf Fawr just to the north of Cefn.

Llwynon House was renovated, for use by the Resident Engineer and his assistants, while offices and huts were constructed on both sides of the Brecon Road nearby.

[10] Workshops were erected to the west of the road, and huts to accommodate the navvies who lived on site were on higher ground to the east of it.

An official inauguration ceremony was held on 28 June 1911, with the Lord Mayor and his party travelling from Cardiff Queen Street to Merthyr Tydfil on the Taff Vale Railway, but it is unclear how they got from there to Cefn yard for the journey onwards to Llwyn-onn.

Ancillary work included the erection of a new chapel, Capel Bethel, to replace the one at Cwm Taf, which would be flooded when the reservoir was filled, and the diversion of the Brecon Road further to the east from below the dam site northwards to Abercar.

Despite the fact that the work stopped on orders from HM Treasury, Nott started litigation as he was unhappy with the settlement he received.

He died on 4 July 1916, but the Scottish civil engineer Robert Brodie was a good friend of the family, and continued the dispute throughout the war.

A workman's train ran from Cefn each morning, calling at the quarry, the dam and Cantref filters, returning in the evening.

Near to Christmas 1921, Priestley, who was now 67, was given a closed Ford car to replace the open model which was not suitable for the cold and wet weather, although it did not arrive until 2 March 1922.

In December 1921, the Corporation also decided they would run a school train to enable children from Llwyn-onn to get to Cefn yard in the morning and back in the afternoon.

The railway was extended into the bottom of the reservoir in early 1922, so that deliveries of stone to clad the inside face of the dam could be made more easily.

Impounding of water in the reservoir began on 23 June 1926, when the Lord Mayor of Cardiff, Alderman W P Francis, partially closed the valves, and Cantref filters were inaugurated on the same day.

In 2020, Welsh Water blocked off lay-bys around the Llwyn Onn reservoir because of persistent complaints about them being used for antisocial behaviour such as dogging.