Fluid operated, oil-cooled brake arrangements are actively being developed for large material hauling vehicles and the like in order to achieve both safe and effective braking.  However, not before the annular dual piston brake arrangement disclosed in U.S. patent application Ser.  No.  522,390 filed Nov. 11, 1974 by P. F. M. Prillinger et al, and assigned to the assignee of the present invention, has multipurpose capacity braking been provided in such a compact and economical package.  The referenced construction not only integrates service braking with emergency braking and/or parking braking in an improved manner, but also is sufficiently radially compact that it can fit relatively closely within a wheel rim while maintaining a high capacity. 
While the aforementioned dual piston brake arrangement is a considerable advancement over the prior art brake mechanisms, the egress of cooling fluid posed certain sizing and constructional limitations on the various members thereof because space is not available radially outwardly of the brake arrangement.  Particularly, in order to permit axial egress of the cooling fluid therefrom, the maximum radial dimensions of the service brake piston and its associated actuation chamber and also the maximum radius of the pitch circle for the compression spring members used therewith were limited.  Consequently, a fewer number of compression springs could be disposed equally around the pitch circle so that they individually had to be increased in length and/or capacity.  These radial limitations subsequently reduced the maximum capacity of the subject brake arrangement for a particular maximum external diameter, and undesirably added to the weight and cost of the package.