Vacuum system

Good night I have a problem and I hope you can guide me. I have a GMC Blazzer 95 4x4 model with 4.3LT 6 cylinders and automatic transmission. My problem is that you notice that the 4x4 does not work that is to say on the Persione board the button that says 4 so and turns off indicating the change and the gear movement in the transmission is heard but the front wheels have no traction, checking I found that the lower part of the accumulator box finds a device as a diaphragm that has a spring but the cover cover is broken. A person tells me that the 4x4 is activated by empty and that he has a pump at the bottom of the capo but does not find any empty hose that is also disconnected by observing in the workshop manual of a user on the page I do not find that empty pump.

See if it carries an electrovalve that is electrically action and take the emptiness directly from the type engine as the Asian 4x4 use and carry a hose towards the front wheels.