Don't be tempted to fill to the top of the dome. Obviously the damper will displace the oil and you don't want it to disperse internally or try to. Take the damper out and look inside. You will see a tube. Oil/ATF just needs to be up to about 1/4"/5mm from the top. When the damper barrel goes into the tube it will displace some oil and so the tube will be filled. They rarely need topping up and are tolerant of being at a low level. As long as you can feel some resistance when you move the damper up and down there is enough oil in there. Move damper slowly. Don't tug it or you will lift oil out over the sides of the tube. With old age the diaphragm itself is prone to splitting. This may be due to too much oil getting on it. When it is split the vacuum won't lift the piston to varying degrees and you will get very poor performance or bad starting. Diaphragms are cheap so have a couple handy. Buy genuine replacements. Change now or wait for problems. Your choice. It's an easy job but be careful not to drop any screws!.