First, Uninstall Clean Master! Here's why:
http://forums.androidcentral.com/showthread.php?p=4435166
Second, you cannot use 100% of the storage available. On any computer OS a minimum amount of free space must be available for normal operation such as writing temporary files, etc. Without free reserve the system becomes unstable, will start crashing or even refuse to run. The exact amount reserved varies but is usually about 200-300MB. At 161MB free you are very low and no doubt hitting the minimum limit.
You must free up some space. To make space you can:
Delete texts or emails. If you have a lot saved on the phone, this can help.
Delete or move photos, videos, documents, etc.
Uninstall app updates.
Uninstall apps.
Clear cache files. Not recommended. This will only make space temporarily as cache files will rebuild over time. Also, while clearing cache files won't hurt anything, they're there to speed up operation and should be left alone.
If you can't move stuff to a SD card, move them to your computer or cloud storage.
Cleaners, RAM optimizers, task killers, etc are unnecessary and counter-productive. Avoid all such crapware.