SIPTU says it has belatedly discovered that there is a mechanical problem associated with certain models of bus in the Dublin Bus fleet.
The union says that despite assurances from management it is concerned about driver and customer safety, and wants an independent safety body to examine the problem.
In response Dublin Bus confirmed there was an issue regarding a braking system on certain buses.
However the company said all the affected components were being replaced in a process that would take ten days.
In the meantime it stressed there was no risk to the safety of customers, staff or the general public.