Thanks.
Yesterday we managed to disassemble the actuator but I did not have time to post the photos. It is quite easy if you find the proper tool to bend out the clamps on the housing (shown on photos).
The housing is fully galvanised so the inner side is in very good condition and the membrane is also perfect.
There is a busing behind the seal to give direction to the piston and that was the sticky part. After removing it a reamer (10mm) refreshed the hole and the piston was polished again.
Now the housing is waiting for having a new zinc bath (the outer surface have some rust) and I have to buy a new seal.
I will post some photos about the assembly in the future.
I am curious. Was there rust on the shaft that prevented it from moving in and out? You wrote ealier that it worked in one direction, so I presume it was not stuck. As you found there was nothing wrong with the membrane.
Usually a bit of WD40 or such will be sufficient to make it move freely. Most problems are with cracked hoses to one or both sides of the actuator, or the vacuum valve.
The vacuum valve can me taken apart and, if you do, it is better to drill completely through the screw holes and install long 4mm bolts with nuts on the other side, rather than the original tapped kind. The tapped kind has a tendency to crack the nylon on the valve.
That had happened on my valve and i had a constant hissing from it, because it was not tight.