Lunar Module Pressurisation

To preserve the LM consumables, the tunnel and LM are pressurised from the CM cabin oxygen via a Pressure Equalization valve built into the centre of the CM's forward hatch, through the tunnel and through a valve built into the overhead hatch of the LM. This latter valve had been deliberately left open since before launch to allow the LM's atmosphere to bleed away as the Saturn ascended. Now it admits CM air (O2) to the LM so that during the coming day, the integrity of the LM cabin can be proven in a vacuum. If, during pressurization, the CM pressure drops to 27.5 kPa (4.0 psi), more oxygen is fed into the CM cabin from the repressurisation package to replenish it and keep it above this figure.

A pressure gauge beside the CM forward hatch lets the crew monitor the pressure difference across the hatch and when this is low enough they can remove it and check the latches, to see if any haven't latched properly and have to be engaged manually. Two umbilical cables are connected within the tunnel to join the two spacecraft electrically. The hatch is reinstalled and the spacecraft prepared for its ejection from the S-IVB. (ap15fj)

[edit] Apollo 15

According to the 1971 Mission Report - "The probe was activated to the retract position and the two spacecraft were hard-docked. At the completion of the docking maneuver, the forward hatch was removed and the latches were checked. One latch was not locked onto the docking ring. That latch was recocked and latched manually. The Lunar Module umbilicals were then attached, and the hatch was replaced."

Scott, from the 1971 Technical Debrief - [To Worden] "Once you gave it the plus-X, I was watching the talkbacks, and they flipped right in the barber pole. We retracted and cinched right on down and heard a good bang on the latch."

Worden, from the 1971 Technical Debrief - "That's very positive. Not only can you hear it, but you can feel it, too, when those latches go. You really know you are there."

Irwin, from the 1971 Technical Debrief - "Number 3 was the one that wasn't latched. It took two strokes."

Scott, from the 1971 Technical Debrief - "Yes, that's right. All of them were locked up tight except number 3. It took two strokes to lock it. Could you see anything hanging from the LM?"

Worden, from the 1971 Technical Debrief - "I didn't see a thing."

Scott, from the 1971 Technical Debrief - "You mentioned the handling characteristics. Sunlight and CSM docking lights must have been okay."

Worden, from the 1971 Technical Debrief - "We didn't need the docking light. Everything was illuminated by the Sun. We didn't have any problems with shadow; no problem seeing the docking target. It was very clearly illuminated. We didn't use the docking lights."