In the field of avionics, it is difficult to justify initiation of new aircraft programmes for replacement of small numbers of niche aircraft. Instead, in many cases, retrofitting existing aircraft is preferred for economic reasons. In this respect, upgrading existing braking systems to accommodate replacement of existing brake units employing sintered friction materials with brake units employing carbon-carbon composite friction materials is desirable. However, when upgrading the braking systems it is necessary to ensure that the integrity of the existing airframe is not compromised by the use of the new brake assemblies employing the carbon-carbon composite friction materials. Consequently, the present invention replaces existing pressure control valves of existing braking systems with an electronic control unit (126) coupled to a pressure control servovalve (128) thereby ensuring proper application of hydraulic pressure to brake assemblies (122). An advantage of this invention is that the pressure profile of the hydraulic fluid is controllable and hence the integrity of the airframe is maintained.