The Shrimps looked set for a rare win at the Globe Arena but their celebrations were cut short when veteran defender Darren Purse took advantage of a mistake by goalkeeper Barry Roche to head into an empty net and earn his side a share of the spoils.
Earlier, Luke Young had given the Pilgrims the lead after three minutes before Jordan Burrow and Lewis Alessandra put Bentley's side on the verge of victory.
A frustrated Bentley said: "I'm disappointed that we couldn't hold on because I really wanted to give our fans something to shout about and we looked close to doing that after a much better second-half show. We got off to the worst possible start and we were poor in the first half.
"We had some strong words with the lads at the break. To be fair to them, they gave us a response. We were much better after the break and scored a couple of goals but once again we couldn't see it through.
"The keeper has made a rare mistake and we have been punished and that is something we have done time and time again here this season. We have to learn that if we give the other team half a chance at this level they are likely to take it."