EDIT: Opps....didn't see UF's post...still working on my coffee.
Hi flienlow, If your bell siphon doesn't stop draining, it could be, something as simple as the water comming 'in', is too fast. You could split the line coming to the media bed, and send some back to the fish tank. Which could help add some oxygen.
You mentioned, adding a reducer to the stand pipe (affnan style). Normally this helps to 'start' the siphon. From experience, I noticed, that this speeds up the 'draining' of the media bed. The extra speed, 'might' help break the siphon.
The other option, is to try adding a snorkel tube. It's just a small tube, that comes out of the side of the 'cap'. How far it goes towards the bottom of the bed, sets where the siphon breaks. Here's a pdf, that explains their use...
http://www.ctahr.hawaii.edu/oc/freepubs/pdf/BIO-10.pdf
Sorry, I was confused by what you described in #1....?
I'll just mention some stuff, which sounds like, you've already done or know....?Originally Posted by flienlow
-If your using a 3/4 inch 'stand pipe', your 'cap pipe', should be twice the size, or 1 1/2 inch.
-The 'outside' bottom of the 'cap' should be at the height of the 'stand pipe'.
Good Luck ...