I suspect TR engines leaked oil when new and by the time they had covered a sporting few miles, just as with Brit m/cycles at shows, if they did not have drip trays under their perfectly dry engines, they were likely to have no oil in them... (-:
You could be chasing your tail here.... I never ran an engine to see if oil might splash out through the dipstick hole, though logic seems to suggest to me that the sump being where the oil is stored and that there is a crankshaft spinning around at engine revs oil dripping down from the galleries in the head to the sump, chances there would be a fair amount of oil being splashed around. My guess is that any one piston returning to BDC might be expected to create some degree of crank case pressure, though I expect that to be countered by whichever other piston is on it`s way to TDC, however with all that metal flying around and splashing oil about some degree of crank case pressure seems bound to be created... ADD to that scenario the possibility of blow-by in a marginally worn bore/piston rings during compression and eventual ignition, then it seems simple to imagine more crank case pressure will exist....?
Again it seems the design engineers of the time expected their engines to wear yet still be in use with perhaps 50k miles covered thus the chances were that they expected crank case pressure to be an issue increasing with time...? That perhaps being the case then to save blowing gaskets and seals venting the crank case seemed the logical way to go...?
Back at the time the engine was designed many manufacturers used a vent pipe in the block which exited the block and just pointed down at the road surface. That the TR unit you have has a pipe that initially points upward before a sort of "U" bend arrangement, suggests to me that at the time it was thought best not to spray oil on to the road, and/or, in a worn engine what were the chances in time it would blow much of it`s oil out through the breather on a long run, and thus risk engine damage.... That being a possibility the design may have been such as any oil being blown into the pipe would have to travel up against gravity, the hope being that the majority of the oil would run back down into the sump...? That being a possibility the convoluted shape of the pipe then acting as a precursor to the air/oil separators more modern engines now have...?
I guess over the years and dependant on usage and maintenance either good or not so much, oil can end up considerably contaminated with sludge which can foul things up a tad. pulling things apart can lead to even more leaks.. The much younger me would remove the oil cap, attach a long tube to the breather output, and just blow through it to check just how much resistance might be felt, not suggesting you should do that, but that is entirely up to you...
As has already been suggested by Grumpy, a compression check will perhaps provide a clue.... being an elderly unit, ideally the readings should be reasonably similar across all four pots, though if one is a lot less than the rest, you may have found a possible culprit to higher than "normal" crank case pressure build up...?
However one pot with reduced pressure could be down to an exhaust valve seat/face issue.... There are tests for just such an occurrence, though perhaps you have more than enough to consider for the moment..?
Good luck, more so in that I have not read over this to check I have not made any blunders.... (-:
Where is Arwyn or John when you need them...? (-:
