A rubber funnel seal will last for decades. However, a siphon only works well if it is able to pull a vacuum. So I decided to replace the funnel seal on a 38-year-old vacuum coffee maker—whether it needed it or not—as directed by the vendor’s service department.
Step 1: Place the funnel face down on a non-slippery flat surface. Use a sharp knife to cut away the old seal. I used the hobby knife shown here with a stiff blade. But an X-ACTO knife, box-cutter knife or utility knife would have worked just as well.
Cut the seal lengthwise, as indicated by the white line, in multiple shallow passes. As long as the knife blade is sharp, you should not need to press very hard. Peal the seal away when it splits, then use the knife to remove any excess adhesive.
Step 2: Smear a dab of dishwashing liquid or similar on the tip of the funnel.
Step 3: Push the new seal onto the funnel stem.
Step 4: Slide the funnel seal to about 1″ from the neck of the funnel and apply a suitable clear adhesive. I selected silicon sealant from Loctite, which is rated for use on rubber and glass. In addition to being waterproof, the sealant is rated for short-term exposure to elevated temperatures.
I used a small brush to apply the sealant directly and evenly to the rubber. While there is no need to apply a lot of sealant, you are better off applying a little more than is needed, as this will eliminate visible air bubbles.
Step 5: Push the seal firmly into place.
Step 6: Verify that there aren’t any air bubbles. If there are, you may be able to eliminate the bubbles by twisting the seal, pressing it more firmly or adding more adhesive.
Step 7: Remove excess adhesive while it is still wet as directed by the instructions. For this silicone sealant, Loctite recommends using mineral oil to remove uncured adhesive.
I removed the bulk of the excess sealant using paper towels and mineral oil, then cleaned any residue off the glass using a lint-free cotton cloth and mineral oil. (Don’t worry, you can remove any dry sealant goo later using a knife blade.)
Step 8: Let the adhesive cure according to the manufacturer’s instructions, and Bob’s your uncle. Your siphon is good to go for another 40 years.