For what it is worth....
I received a Rider Ericsson R01 pumping engine for Christmas.
The purchase was at the 'sale' price of $199 plus beaucoup $$$
for shipping. It is a nice looking engine and, once I got the
hang of lighting it and keeping it lit at a modest flame, it
ran well. Fast, even. I tried the priming to no avail. Worst
of all was trying to get the little primer cap screwed back into
the priming cup while it is running. It made me feel very clumsy.
I did try a bit of steam oil and it started pumping but that is
cheating.
So, with it running, I used a hypodermic needle with silicone
tubing at the end to fill (and keep filling) the primer cup. When
water started to pump through, instead of trying to get that tiny
cap threaded into the primer cup I took a bit of the silicone tubing
(with one end flame sealed) and stuck it on the priming cup. VERRRRRY
easy. That is my trick and advice. Works for me. I added several pics.
Good Luck,
Wayne