Artwork depicting the EVA in question, NASA, via wikimedia.org
On the way back an EVA was performed, and since there wasn't an airlock, the Command Module was depressurized.
Was this feature only added for Apollo 15?* Did it vent to the outside, or did it refill the tanks of the Service Module?
* If depressurization was always a feature, what other situations was it designed for?
The CSM was always able to be depressurized, since the contingency procedure for getting back from the LM to the CSM, in case the docking mechanism would fail to latch or the doors were unable to open, was to go around via the outside (i.e. open the LM hatch and CM hatch and transfer via an EVA):
An Apollo contingency transfer is required if the lunar module (LM) cannot be docked to the command module (CM) after lunar-orbit rendezvous.
(from APOLLO CREW PROCEDURES, SIMULATION, AND FLIGHT PLANNING)
(The extravehicular transfer is contingency procedure 5.3.14 in the Apollo Operations Handbook (pdf) in case IVT (intravehicular transfer) is not possible)
The depressurization was via a valve that just vented to outside:
The main, or side hatch has a depressurisation valve built into it. It is operated by releasing a handle and winding the valve open.
(from Apollo 15 Flight Journal)
To open the hatch, the cabin must be completely depressurized by opening a cabin relief and dump valve on the hatch.
Apollo 9 also did a EVA - The plan was to test using EVA to get from the LM to the CSM, although they didn't actually do it.
From Wikipedia:
Scott stood in the CM's hatch; both men photographed each other, and retrieved experiments from the exterior of their vehicles.
