This is an endemic problem with Grohedahl back inlet valve washers. For those of you in Australia, they are supplied by your plumbing supplier through Argent based in Brisbane and cost $9.00 each. It is Argent Part Number 43733. (Don't let your supplier sell you the whole assembly that costs more than twice that amount.) Of course, it's very poor design indeed: the washer needs to be of much thicker silicone rubber than it currently is. I have thought about this problem for years and finally found the solution, I think. I don't know much about the 'pimple' experience. My problems have always been a small tear in the silcone rubber 'dome' washer which houses the red (in my case) or blue solid plasic insert which does the actual sealing. The silicone rubber washer tears, of course, with the repetitive movement backwards and forwards of the solid plastic insert. At the moment, I have a badly torn silicone rubber seal that I repaired with Selley's Marine Sealant. A small tube costs around $11.00. It cures in 24 hours. And its been working perfectly for well over a week with no sign of leakage whatsoever. So, what I suggest you all do is purchase a new washer and run a bead of Marine Sealant around the first 'step' in the washer - that's the only part of the washer that can fail as far as I can judge. With that added silicone 'bulk' at the very point of stress, I see a very extended life for a new washer. I bought a new washer, but my Marine Sealant-repaired torn washer is performing so well that I shall leave it on. In the meantime and subject to comments below, I will run a bead around the first lip of the new washer and wonder when I might need it!
But there's small catch with what I propose... Selley's Marine Sealant once opened is impossible to extract from the tube even though it will feel soft within the tube and despite tight replacement of the top (in my case) within one (1) minute. I rang Selley's Technical Assistance (Charlie) and he affirmed that it was a 'tube problem' that Selley's recognises, but haven't yet pinned down. I was able to fix my torn washer with a minute amount before the problem emerged. It was when I went to run a bead around my new washer a couple of days later that the problem emerged. I took the tube back to the seller (Magnet Mart) and received a full refund. And here's a laugh: after relating my experience with the tube and my conversation with Selley's, a young man at MM thought 'I'll sort this out, no sweat'. After all, there was the tube full of 'soft' sealant. So, off with the screwtop, applied all his weight, burst the tube at the bottom, covered the counter with Marine Sealant and, with an amazed look that I shall long remember, realised that not a single bit had come out of the top!!!
Finally, it might be that something like Gutter and Roof Sealant is sufficiently strong. I can't see why not. The purpose is simply to 'bulk up' the stress-prone silicone rubber.
Good luck to all 'back inlet washer fixers'
Norman
Norman, November 2007