Had the same problem with my 2006 Electra, the faster you go the more it will leak. The pipe that you are discussing is part of the pollution control system. It runs from the PAV unit to the cylinder head to interduce oxygen to the exhaust gas so that the combustion within the exhaust is more complete. This leads to lower emissions. The problem that I found is that the vacuum that is created at the PAV unit caused by the exhaust gases, (venturi effect, the same way a carburetor works), is strong enough to draw oil out of the oil overflow can located under the seat. This unit collects the oil that is voided from the crankcase as pressure within the crankcase builds as piston speed increases. That is the main reason why it's important not to overfill your crankcase with oil. The oil pools in the bottom of the airbox soaking the air filter and limiting the air flow through the filter. The problem was so bad on my Electra that I was using oil at the rate of 1/2 quart every 100 miles.
The simple solution is to disconnect the large rubber hose from the top of the PAV unit. By doing this it will not draw the air from the airbox, this is how the Bullet Classic is plumbed. The added benefit will be no more oil soaked filters and other related problems, very little if any oil is drawn into the airbox.
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I also pulled the large hose out completely, cut it, and redirected it from the air box to the drive chain in front of the chainguard to direct any excess oil from the airbox to the chain to help oil it.
If you don't mind the related problem of the oil use and soaked air filter, you can fix the leak problem by obtaining a brass washer of the proper I/D from your local hardware store to replace the copper sealing washer with. This washer can be obtained with a larger surface area so that the shoulder on the connector which screws into the cylinder head can form a better seal. Another hint is that you need to keep the hose clamp on the hose loose until last. Make all the other connections at the cylinder head first for proper alignment.
On another note you took a nice clear photo of the engine, I think I'll frame it. Beautiful isn't it.
Happy wrenching!