1979-up Sportster engine crankcase breather valve assemblies all have a tiny oil/air drainback hole installed on the outside of the breather valve.

• The breather valve has a built in pocket in the cam cover (see pics below).
• Air / oil mist enters the pocket and flows past the valve.
• Just past the valve (between it and the vent outlet) is another smaller chamber. That is the final separation chamber for air and oil.
  • Oil droplets fall out of suspension there and down into a small hole between the valve and the cover. The drain leads back into the (cam chest - pre 91) or (rocker box - post 91).
  • Air in that small chamber is sent out the 10:00 vent hole in the cam cover (pre 91) or head vents (post 91).
• The bottom of the separation chamber where it says “reed” in the pic below is not really the bottom of the chamber.
  There is a space (final separation chamber) behind the reed (between the reed and the cover).
  That is where oil droplets are pulled back into the gearcase that make it past the reed.
  Vacuum in the bottom end pulls oil from that chamber back into the gearcase.

Air and oil mist gets blown into the breather valve.

In the cavity behind the reed, oil drops out of suspension and drains back. At the same time, the air is purged out the vent hose.

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)

98 Rocker Box Breather Compartment. 4)