i tried to make a new one for my 350 from a piece of truck inner tube and it didnt work. i think due to the size (contact area) of the diaphram there wasnt enough movement because it was too stiff because of the thickness for vacuum to actuate it for the required movement to shift gears.
i was going to try some roof rubber and with all the hasle to make a new one i threw in the towell. some pneumatic diaphram material is the right ticket if you can find some.
