When low water pressure becomes a violation
Low water pressure can be a habitability issue if it is severe enough that faucets, showers, or toilets do not work properly and the landlord fails to fix it. Mild or occasional pressure changes usually are not a violation.
The law requires plumbing in good working order, so water pressure so low that you cannot reasonably bathe, clean, or use the fixtures may cross the line. The cause could be a plumbing blockage, corroded pipes, or a supply problem — generally the landlord's to diagnose and repair.
Report it to your landlord in writing and describe how the low pressure affects daily use, since that helps show it is more than a minor annoyance. A brief, temporary dip is different from ongoing pressure that makes the plumbing hard to use. If your landlord ignores severe low water pressure, a free review can help you understand your options.
This is general information about California tenant rights, not legal advice. Every situation is different.