Well, you could of sucked in dirt or debris into the reservoir depending on how long the cap was off, but I think you'll be ok. Just mold a piece of tin foil into the shape of a cap for now until you get a new one.nyluder22 said:First, I didn't want to start another post. So, yesterday I forgot to put the power steering fluid cap back on the reservoir. You know the small red one. Would that cause any problems. What should I put on there until I get another cap?