BM 3.9 sp3 on nw65 sp2

Untill today, had one large traditional volume (50gb) ... when the
volume would fill up, the proxy would crash and then it was forever to
get it back up.

Now, 2 4gb cache volumes, traditional, no sub, no compression, no
salvage ...

The question is:

it seems that cache aging settings should prevent the volume from
filling up .. yes? Is something broken, or am I missing something? I
don't see any value in the "border manager setup" tab that explicitly
says "delete stuff when the volume gets full", or "don't let the cache
items sit on the disk longer than X days/hours.

How do I configure these 2 (much smaller) volumes such that they do not
run out of space like the old big one (but much more quickly presumably)?