1979-up Sportster engine crankcase breather valve assemblies all have a tiny oil/air drainback hole installed on the outside of the valve.
The drain hole has a couple functions when it's all said and done.

1. Engine vacuum pulls the oil that gets past the valve back into the engine thru the drain hole.
2. The drain pulls in a slight amount of atmosphere to assist in overall vacuum control.

The MoCo had these little things ironed out pretty well. They predetermined the drain size so it would act as intended and not derail the pressure.
1979-1990 engines have a hollow runner with a plug at the 6 O'clock position with the cam cover installed.
There is a dimple on that runner with the oil drain hole drilled into it.
See pics below;
The 77 breather compartment in the cover below has oil drainback hole in the cover (wouldn't work if it was there anyway).
The 81 cover was fitted with the rubber breather umbrella upgrade (26909-79A) which is not important to this discussion.
But, see the bossed area below where the breather is usually installed. There is a runner going down with a dimple and a tiny hole in the dimple.
That is the oil drain hole. It is a very small hole and oil just can't drain from there too fast without vacuum to pull it out.
The 85 cover has the breather baffle tube system but the principles are the same and it doesn't appear the drain hole size changed.

77 Cam Cover Breather Compartment (no oil drainback). 1)	81 Cam Cover Breather Compartment (w / rubber umbrella valve upgrade) Oil drainback hole in dimple. 2)	85 cam cover w / baffle tube system. Oil drainback in dimple. 3)