I've been looking, thought I still had an Elan manual, but can't seem to locate it. Am I correct in that the front part of the frame was used as a vacuum reservoir? If so, there shouldn't be too moch instant demend to operate the headlamps, so the line size shouldn't be too critical. The main problem we used to have wase leakage, could be in the pots, the frame welds, or most likely in the one way valve, altho a split plastic teewas not uncomon, as in the plastic tee in the fuel line between the weber carbs.
Almost any one-way checkvalve ought to do the trick. You might check out irrigation sprinkler, suppliers or plumbingsuppliers. Perhaps a fuel line check valve if the spring pressure is not too great.
Good luck.
Dave T
bluespit@attbi.com