Thanks for the useful info Simon. I've a similar oil leak problem with my BSA Sloper, which has a fairly useless felt seal behind the mag pinion.
I'll check the breather as you suggest - if i can find some suitable tubing.
I've taken my pump off and cleaned it out. Here's a clip of it: https://youtu.be/r5cPChWzAr0
Also a photo of the pump mounting face:
What's the socket head bolt (indicated in red) for ?
Since this is the first time I've seen one, any comments would be welcome.
Haven't removed it yet, but the current paper gasket seems incredibly thin. The thinest gasket paper I could buy this morning is 0.4 mm - is this o.k. for this purpose. I assume that you smear engine oil on both sides of the new paper gasket when refitting ? Also, do I need to prime the system prior to starting ? if so, how ? Cheers, Bevan