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Actually, 4 maxi brakes is an option. The majority of OTR tractors only have maxi's on the front drive axle. Mine is that way. They put them on the front, because unless the interlock is engaged the front axle is not a drive axle. By having the rear axle free of parking brakes you can push/pull start the truck in an emergency.
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I agree that 4 maxis is an option. However, the vast majority of the ones I see do have 4 maxis. Almost nobody orders only 2 any more.
Also, all 4 wheels are driving whether or not the interlock is engaged. All tandems have a power divider that does just that, divides power between the front and rear. The interlock operates just like a side to side diff lock except it is front to rear locking the axles together, disallowing differential action between them while it is engaged.