I just managed to get mine working, it is a bit of a faff. The problem was the sponge in the vacuum cap had disintegrated.
If you remove the vacuum unit and push the plunger in then put your finger over the pipe stub and the area over the metal strip holding the sponge the plunger should not come out, it should be held by the vacuum in the unit.
If this works all is well. If the plunger comes back out then you have a leak, poss from the diaphragm.
If the diaphragm checks out to replace the sponge carefully remove the metal strip and you will see the sponge disc and rubber disc, this is a one way valve, it lets air out when the piston is pushed in and should not let air in.
If the sponge is toast or gone you need to replace it, I used a tiny piece cut from the foam of a kitchen scourer pad. The amount you use is crucial - too much and the valve wont blow off and the piston will come right back out, too little and the valve wont seal.
What you want to happen when you have fitted new foam is to be able to push the piston back with your finger over the pipe stub and the piston to stay back for as long as you hold your finger over the pipe stub.
Then when you ease your finger from the pipe stub the piston will come out, this mimicks the bleed off valve at the steering column.
