I have recently buy a PR 2 sub domain which is 6 year old and then developed a website on it. After few month I noticed that domain also have sub-domain which have PR 3 with the name of "forum". I know, today PR have no more worth. But want your suggestion, is it beneficial to permanently redirect PR 3 sub-domain to main domain. Sub domain have few backlinks. Should I get any improvement in ranking for main domain? Niche is totally different from previous one and I also don't have enough time to run a forum.
You will get no benefit, sorry. Google is smarter than that. I used old domain with old back-links - and they did me only bad. People came by that links and closed the browser. It skyrocketed my Bounce Rate. Do you need it?