When I removed the MC I decided to install the C clip (it is actually a snap ring). What I discovered was it was quite loose in it's grove as if it was too small.
So that led me to suspect there was something wrong with the snap ring. Quick experiment showed it was not a good spring in that it did not return to it's same diameter after being narrowed with the snap ring pliers.
Decided it was time for a new ring. The new ring seemed to hold it's shape after deformed to install and importantly fits tightly into it's grove.
So far it's holding. Only about 2 miles on a test drive but lots of braking. Maybe I just got an inproperly tempered snap ring in the kit??