Brake problem chev. K-1500 4x4 With ABS brakes

You brake reservoir is set up backwards so the small front is actually for the rears and the big back res. is for the fronts, so what i would do is put the top on the reservoir like you would be driving have a buddy sit and start truck and pump the brakes while you follow the frame rail your lines are on and look for leaks. If everything is dry look pull your rear tires and drums and see if there is any moisture inside it should be dry so if one or both sides are wet replace wheel cylinder