Have had a couple of dizzies recently where the canisters haven't held vacuum. I'd agree it's just a small sample I've had, I'd guess maybe a 1/3 haven't been working, which is a noticible fail rate. Short of a miracle, you're not going to find a genuine new replacement and used ones are just as thin on the ground. Yes, you can occassionally get a cheap dizzie with one attached, but unless you're aware of part numbers, it's a bit more involved than that as they do have specific settings, plus there's always the quite high risk of it not working either. There's sellers offering new repro ones, though they seem to be creeping up in price, but they're also still a bit of an unknown as far as compatibility goes. If they were cheaper I'd probably get a few to test or even experiment with adjusting to a desired spec. but I'm not made of money! So if you can fix your original it gives you another option. First thing is to get it off the dizzie, just loosen the 2 x screws and it'll move away from the main body. All the VW canisters I've had have the arm of the canister attached to the inner plate by simply pressing the hole over a stud... it'll just push off and on. Be aware though that there have been some non-VW ones that have a small circlip holding the arm to that inner plate instead, that's a dizzie apart job unfortunately. There's likey other reasons, but the two causes for failing I've found are the metal can has either a pin hole due to corrosion or the inner diaphragm is punctured. Before you start taking things apart needlessly it would be worth while ruling out the former first, it's an easy fix using Araldite or similar. If it is potentially the diaphragm and you do have to go in, I've found grinding the overlapping flange around it's circumference works best / gets the tidiest end result. Did try prising the overlap away instead, but couldn't get it to move, though you might fair better. You'll end up with three parts, the canisters front and back, plus the overlap (that's now in the form of a ring). There's no reason to put that ring back at the end, I just did it for appearances. The old diaphragm will have just been sandwiched in place, so everything'll part company easily with a bit of care. Had toyed with the idea of drilling out the arm where it connects to the small metal center pieces, but wasn't convinced I could get it back together and it still have the strength needed not to part company when vacuums applied. Fix was to cut an under sized hole in the center of the replacement piece that stretches over the rear metal part, then cut the old diaphragm down to become a flap / seal that's glued to the new one. You have to have the new material on the side I've shown, remember the vacuum will be pulling in one direction (the spring in the other) and it needs something to act against. if you for example glued the replacement to the spring side face of the old one it could potentially just separate under vacuum, leaving the arm where it was. The original diaphragm material seems to be a rubberised cloth, amazingly you can buy the same stuff and in different thicknesses, the one used below is 0.25mm to match the original. Reassembly is just gluing all the different layers back in order (don't forget the spring, that'd be sad!), I used Loctite 406. The canister shown here... I have another working one that I was able to check the start / end vacuums against and they matched one another almost perfectly. If there weren't another to compare against you'd be unable to confirm its back to original spec., I've yet to find any definitive lists or charts of what a given cannisters range is. Having said that the vacuum start / end points are dictated by the notch that's been cut in the arm, it's size / location will vary from canister number to canister number but as long as it's not obviously worn I'd imagine you could be pretty sure things are back to where they should be. Maybe if you used a thicker diaphragm material than the original, that resisted the vacuum more, it might influence things, but can't be certain. The amounts of vacuum involved are very small, even using a low calibrated gauge it doesn't take much to go from the start to finish point. The reason the canister is in primer, it helps identify whether or not it's a leaking body or diaphragm by temporarily sealing pin holes that aren't immediately visible. It'll get another blow over just to finish it off some other time
