What year is your 420G.
1967 and some 68 models may have the earlier Dunlop system fitted
here is a picture of it
Its easy to see the difference, booster is different having a band that retains the clamshell. Master will have part of the engagement piston protruding from the master, while the later one , the piston is retracted and restrained with the cylinder.
What manual are you using?
Post some pics, that will be a better idea than a “passion fingers” routine
Putting them back together is surprisingly difficult
Make sure you take many digipics that show the orientation of the seals in the master cyl. If you place one backwards, it wont work. I advise measure the “length” of each seal with a vernier. If they are too long, the prevent the rear port from working.
That happened to me, there are 2 sets of MC seals which are very similar
If the crescent shaped metal parts that are retained to the caliper with 2 screws, it common for them to be broken, good luck if you have a spare set, I also have some spares, last time I just JB Weld then together and scrwed back in, they still do the hob like that
I have to do my fronts, and am looking for my spare front calipers so I can dismantle them also, I have noticed, that as per the parts manual, the 66MK10 has slightly different caliper bodies from a later 420G, I believe the internals are the same
I shall post some pics later
To get them apart, I split the caliper, soak them in turps/atf mix, knock the pistons all the way in, fit tight bleeders to the caliper halves, connect a caliper hose, and pump out each individual piston, restrained by a g-clamp, using the cars unboosted brake hydraulics