My guesstimate is that as I got it it needs about 4 CFM. I've adjusted the airflow down temporarily until I can get better air.
Lots of trial and error with the probe this weekend. The cable has no visible identifier. It is sheathed in 3 overlapping lengths of heatshink, and underneath the heatshrink is that gooey black tape wrap that you often see on wiring harnesses. Underneath that is the Renishaw cable. Anyway once I pulled the tape from the probe end I noted that the heatshrink and tape did not extend totally into the conduit clamp. The conduit clamp itself is corroded, especially the 'bucket' that the conduit is supposed to sit in.
Eventually during my trials the probe completely missed a trigger, so there's something else going on, possibly mechanical. Carefully rewiring to remove the last 2" of the cable made no change to the behaviour, and I later saw another miss when testing with my hand. I think I'm going to have to get inside the probe to see what's going on. I managed to use it to manually set some tool heights though, so not all is lost.
Wiring-wise, it looks like just red and blue are connected internally. I assume from the schematic that they go to zhome and xhome signals. The cable ground shield is effectively connected to the probe housing. Are yellow and green simply floating? Or grounded at one or both ends? That's not clear to me yet.
I found that bulb for the power switch and I think I have a suitable replacement on its way from digikey. It appears to be a T3 1/4 9mm bayonet blub, 24-30V 1.2W. I'll post the digikey part number once I know for sure.
I know that the x-axis encoder was replaced, so that explains the missing error compensation. I wonder if it's possible to generate a null compensation file for X so I can keep using Y and Z compensation. Anyway, it might simply be one more reason to retrofit linuxcnc earlier rather than later.