Well, I have learned a thing or two today.
Does this seem vaguely familiar, in a way?
Turns out, my guess that the servo's off a later truck, is spot
on. The screenshot above is from a copy of the Bedford
Truck & Coach brake service training manual (TS1087) for trucks
from 1973. And it also turns out, I was not missing any parts from
the actuation valve in the servo, like I originally
thought. ...though, by now, I'm sure they have gone missing amongst
the toolboxes & workshop moves. 
So that's neat. Assuming I've not lost the bits, I can reassemble the proper control valve, and hopefully it seals!
(I've also learnt that there's a variant of the same system with a spring park brake, which works by putting the spring park chamber on the rear axle, pulling on the same relay arm that the handbrake cable does on the RL. Neat!)
glenanderson
A spring park brake obviously gives a layer of failsafe in the event of vacuum loss, which is in general a good thing and is great on something that gets a lot of use. However, leaving the wagon parked up for extended periods with the rear brakes hard on is a recipe for stuck brakes. I’m fairly sure that is why the military Bedford MK and MJ models retained a separate handbrake despite having air-over hydraulics right up until they were phased out in the late 80s. I’m not saying don’t fit a spring brake, just be aware of the downside.
Oh, I'm not planning on fitting a spring brake, but it's neat to know it was done (in various ways) and that I could do it if I thought I needed it. I could certainly see them sticking in place if left hard on like a spring chamber would do. Though I wonder if the military's priority list involved "can still drive away with a hole in the air system", more than anything?
In any case, the manual covers a whole range of variants, both vacuum and air, with and without spring & trailer brakes; which is interesting to see.
Using up some older footage:
CaptainSlog
Just read the whole adventure - love these RLs, we used to use them in the Rhodesian army in the 70s, they would go anywhere, slowly! I never saw one broken down other than if it hit a land mine.
Yeah, it's hardly going to set the world alight when it comes to
performance! Might be highly entertaining to find somewhere to put it
on a dyno if I ever finish, though! 
One thing I've noticed, now
that I'm allegedly a truck mechanic, is that... it's very stoutly
built, for the 'plated' weight! More like an 18 tonner than an
8-and-a-bit!
Still, I'm not planning on running over any land-mines, regardless of
how stout it is... 
Not much left of that one.
varelse
The gunners hatch is a neat addition
I think it was in the process of being phased out circa 1958, so mine
has it but it's capped off and doesn't have the little mounting balls
for the gun cradle. All it'll have to hold up now is the beacon
In unrelated news, I was looking at the 1958 handbook again, going to see what they listed for the fuel economy back in the day; and I managed to find an error in the handbook. It lists the target fuel consumption as 10 MPG (imperial) or 35km/litre. Now, 35km/litre translates across as 2.86litre/100km, which is nothing short of miraculous.
What does 10 MPG convert across to, though? 28 litre/100km, or ... 3.5
km/litre. Oops! Someone moved a decimal point somewhere! Now, while
this is the consumption figure for the older, lower-compression, lower
power engine; I'm not expecting miracles out of the current engine.
For sake of some comparison, I found a review of an 18tonne Euro 3 DAF LF with the 6.7 litre, and loaded up to its plated weight it averaged 19.8litre/100km.
It will be quite painful filling up the tank on the Bedford, I'm sure. (Just got to think of it in terms of smiles per gallon, because the alternative is lots of crying.)
Frankenhealey
First time I took the Goddess to Goodwood, a round trip of 160 miles, it cost me IIRC about £150 with the spare jerrycans. The cummins is at least twice as economical and one-and-half times the torque.
Yeah, at current prices I'd be expecting somewhere in the region of
£150 just to fill the tank; and how far I can get on that still
remains to be seen. At the moment, it's really good on fuel... doesn't
use any!