[Glider]

There's no crankcase pressure up there, hence no need for breathers.

If you've got leaks it's because the surfaces aren't flat, the gaskets are no good (cheap stock paper gaskets) or you're using a poor sealant.

Check that all surfaces are flat, smooth and clean. Then wipe with lacquer thinner to remove all oil, then apply a thin film of Yamabond (or Hondabond) to the head surfaces only.

If you've got leaks from the inspection caps, use new stock gaskets but apply grease to both surfaces before installing them. Or make your own gaskets from cork gasket material (that's what I do), or use 0-rings with a light coating of grease.

Nothing makes a Triumph engine builder look sillier than breathers on the rocker boxes!
BSA's however will benefit from a rocker box breather because there is crankcase pressure there.

 

[Glider]

Okay Jack, you got me. I know the oil gets down there through a couple of little holes in the tappet blocks, so there is some connection between the boxes and the crankcase, but we all know that there's very little pressure up there and that if the boxes are leaking it's usually due to other causes.
I wonder if putting breathers up there and allowing pressure to flow up through the tappet blocks might hinder the oil flowing downwards?

+1 on the one-way valves. Modifying the engine to breathe through the primary, like the later models, also makes a big difference.