You have to read your thesis to prepare for the viva. Cover to cover, everything, don’t skim. It will help a lot.
If you re-read though, and re-read again, there’ll come a point where it becomes a passive activity. You do it for the sake of doing it. Your attention is on other things, and while you might feel good for taking a look at the book again, you’re not actually getting any benefit.
When that happens, switch gears: go back to being active. Annotate your thesis. Make a list of your most important references. Create some summaries to help you find the heart of your research. Have a conversation with a friend about your results. Ask your supervisor to run through your methodology and throw in some left-field questions.
Effective viva prep is active.