small update as I've managed to narrow things down.

the following will NOT work

[realms]
kdc=server1
kdc=server2
kdc=server3
kdc=server4
master_kdc=server1
master_kdc=server2
master_kdc=server3
master_kdc=server4

when server1 and server2 are down on the network. server3 and server4 are up and functional

but the following WILL work

[realms]
kdc=server1
kdc=server2
kdc=server3
kdc=server4
master_kdc=server1
# master_kdc=server2
master_kdc=server3
master_kdc=server4

So the problem seems to be with having more than one non-functioning master_kdc ??! One seems to be ok but more than one and kinit chokes.