air paths from direct high pressure spray wash during scheduled vehicle cleaning by deflecting and reducing the force of the spray with continuing reference to fig5 the interior of end cap 90 defines three concentric plenum chambers including an inner plenum chamber 96 which communicates with outlet vent path defined by duckbill valve 80 an intermediate plenum chamber 98 which communicates with the inlet vent paths defined by the radially disposed apertures 68 and an outer plenum chamber 100 which is in direct communication with the surrounding atmosphere through a plurality of radially spaced apart side slots 102 formed in the outer peripheral wall 104 of end cap 90 the inner plenum chamber 96 communicates with the surrounding atmosphere through a plurality of axially disposed exhaust ports 106 which extend through the domed end wall 108 of end cap 90 the inner plenum chamber 96 is isolated from the intermediate plenum chamber 98 by radially inner annular wall structure 92a and the intermediate plenum chamber 98 communicates with the outer plenum chamber 100 through a plurality of radially spaced apart slots 110 which extends through the radially outer annular wall structure 92b during wheel rotation side slots 102 serve the additional advantageous function.