I just discover this post, sorry.
In order to reduce the disk space consumption of the cache I recommend running a purge periodically (every 2 or 3 days). A purge deletes the cache of pages that no longer exist or whose context no longer exists. This does not affect cache performance. You can perform a purge manually via a button below the cache statistics table, or by scheduling a CRON job with the URL displayed in the "API (CRON URLs)" menu of the module configuration.
When you have no choice you can reset the cache like this:
TRUNCATE TABLE jm_pagecache;
TRUNCATE TABLE jm_pagecache_contexts;
TRUNCATE TABLE jm_pagecache_details;
TRUNCATE TABLE jm_pagecache_bl;
TRUNCATE TABLE jm_pagecache_mods;
You can also safely delete files in /var/cache/pagecache
Be aware that this is not to do periodically, only if you are blocked.