I had to disconnect one of my 991's radiator's hoses. I caught most of the coolant to pour back into the system when my work was done. But it is not obvious to me where the air bleed valve is. There must be one, right? How else are we to get all the air out of the system? Anyone know?.

 

Bleeding the system is a long and arduous process.
I had the coolant changed at 50k.

It took a specialist garage 3 hours to bleed the system.

There is a Porsche tech computer program that monitors and assists with this process including warm ups, cool down, warm up and engine revving

I do not believe that there is a bleeder valve and (gag) I suggest that you have the dealer perform this service for you.

 

In doing a search on the net is seems there's a bleed valve on the coolant bottle that you open up and then drive around for a day or so then top off coolant when done.

I've bled my older vw's and even my bike by opening up the coolant bottle and running the engine until I see a solid stream of coolant reentering the bottle. Top it off, close it up and good to go.

 

To add to what rnl and CSK contributed... Bleeding the coolant system is possible using the bleed valve that CSK mentions, which is a vent clip on the coolant reservoir. However, performing the bleed is something that Porsche wants to keep a mystery and sounds more complicated than it needs to be.

Looking at the WM (190107 for those interested) for bleeding the cooling system:

1. Connect a battery charger
2. PIWIS Tester II 9818 must be connected to the fuse carrier in the passenger compartment to the left
3. Switch on ignition and Tester
4. Select the vehicle and then select DME in the control unit search
5. Switch to the Maintenance/Repairs menu and then select funtion Bleed Cooling System
6. (And this is the funny part! ) Perform the bleeding process in accordance with the specifications and steps displayed on the Tester.

#6 is funny to me, because the "official work manual" for performing the work just tells the tech to follow the Tester!