Yes, there _were_ 5/16" sized ferrules to fit 1/4" tubing, they were
original on our cars. Unfortunately, I don't know of a source for them
(and neither do Moss and TRF).
One solution may be to take two new 1/4" ferrules (available at any
hardware store), cut off about 1/3 of each one, and solder (or
super-glue) the two halves together. Another that might work is to
carefully file the first thread off of the 1/4 x 5/16 fitting, so that
it can go deeper into the pump fitting.
If someone else has a better solution, I'd love to hear it !
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