brakes 20 22 because there are no fluid separators the rear valve arrangement 58 has release valves with different normal positions than the front valve arrangement 56 more specifically the rear valve arrangement 58 includes a normally closed rear right apply valve 58 a a normally closed rear left apply valve 58 b a normally closed rear right release valve 58 c a normally closed rear left release valve 58 d and a normally open rear cross tie valve 58 e as with the front valve arrangement 56 the rear valve arrangement 58 could include any suitable arrangement of valves for controlling the flow of fluid to the rear brakes 20 22 other specific valve arrangements which could be made include the substitution of a three way valve for a pair of apply and release valves such as was done in u s pat no 5 941 608 to campau et al the rear vehicle brakes 20 22 are normally operated by application of pressurized hydraulic brake fluid thereto from the normal source 12 via the high pressure line 54 and the rear valve arrangement 58 and selectively releasing pressure from the rear vehicle brakes 20 22 via the release valves 58 c 58 d which are connected via the low pressure return line 59 to the fluid reservoir 32 the rear fluid conduit 30 connects the rear vehicle brakes 20 22 with the backup source 14 the rear isolation valve 30 a is disposed in the rear fluid conduit 30 for selectively preventing the flow of hydraulic brake fluid between the backup source 14 and the rear vehicle brakes.