From what I have discovered so far, it seems that the air actuator (servo) might not have a diaphragm, but be a piston type.
Of course this depends on the age of your vehicle and actual model, but '330' TKs are coming up as piston type, and a repair kit is reasonably priced (IMO):-
http://www.ebay.co.uk/itm/Bedford-TK...item2310bf9064
From:-
http://stores.ebay.co.uk/Bedford-TK-...4.l1581&_pgn=2 (shows the early 220 diaphragm repair kit).
Some suppliers of Bedford parts:-
http://www.powertrackbrakes.co.uk/contact.html
http://www.bedfordparts.co.uk/contact.shtml
This is a brake master cylinder, but for a heavier model than yours, and I don't know whether it would fit:-
http://www.ebay.co.uk/itm/Bedford-TK...item2313e96950
It says KG/EPR which is heavier than your KB(?)/EJM - Maybe your model is KD? (I wish I could remember the 'new' designations - ie the third letter (M) in your EJM). The third letter (which indicates the allowable maximum weight) is in the sequence from A rising to Z (though the extremes A and Z weren't used IIRC), so a letter M is lower GVW (Gross Vehicle Weight) than a letter R (which I know was a KG). Third letter V was a KM (16 tonner), X was 22 tonner (and 24 tonner, too IIRC).
It seems (from the interweb) that the third letter G was a KB, so maybe yours is a KD (I don't think it is a KE or KF though it could be if it was coded with small wheels - which were usually fitted to KB as standard.
I've just found a 'translation' list:-
Third letter:-
F=KA
G=KB=TK570
L=KC=TK750
M=KD=TK860
N=KE=TK1000
R=KG=TK1260
T=KH=TK1470
Which seems to confirm your vehicle as a TK KD/TK860 (your allowable weight (GVW) should be 860Kg?
Sorry if this confuses you, but when they changed from simple KA/KB to EOF/EOG I lost interest!
Here's another translation list:-
http://avi-ltd.co.uk/id18.html
Which might mean that the digits that you thought were 2000 or 2060 could be 2BCO (with the 2 being the wheelbase designator).
I don't remember what the next letter indicated, but the CO meant (IIRC) supplied with a fully-built cab (a letter Z indicated no cab).
It's been 30 years since I was involved in such matters, so it's no surprise that I've forgotten some of it!