I only know it how it works on a 68 backing plate don`t know the differnce to before.
When you install a typ-3 wheel cyl. in an bug plate the air relief valve sits UNDER the supply line. So the air doesn`t come out.
One way is to remove the backing plate or wheel cylinder seperate and press the air out in upside-down position.
And I needed to notch the 40mm brake shoes a bit where the lay on the backing plate...
It seems that here are typ-3 weeks?!