Howard
As others have said this is almost certainly the master cylinder seals , and as they are not sold separately these days , you are probably going to have to change the entire unit. Not a difficult job in itself , just take you time bleeding the brakes thoroughly until you are really happy that all the air is out , and then do it again about a week later just to check. A pressure bleeder will help , but it is not essential , and the old small length of runner tube and a jam jar will work just as well.
The 1970 +8 did not have a brake servo at all. But I think it was the first at MMC to feature split circuit brakes and hence the more complex master cylinder with an outlet pipe to each circuit.
What happens to cars that are not used regularly (and especially to those laid up over winter) is that the central rubber seal "relaxes" and then gives the impression of a long pedal even when everything else is adjusted correctly. I don't think it is worth trying to re-set the seal (although I am told some do it) because you really do want your brakes to work and inspire confidence when travelling at +8 speeds.
