Hi - Hoping someone may be able to help me?

12 months ago my C180 W202 (1996) 'automatic' developed a fault. When I put into reverse gear and started to move backwards the brakes would partially engage. I was still able to reverse (just about) but obviously with the brakes partially engaged it was a struggle and I needed lots more revs.

Note: If I shifted into neutral I could push the car backwards without a problem.

At the time a simple adjustment (loosening) of the parking brake cable solved the problem. For the past 12 months everything has been fine, that is until a few days ago when the problem returned. Unfortunately this time the same remedy hasn't worked.

Locally someone has suggested it's a gearbox problem. Is that likely? If it's a gearbox issue it just seems strange that adjusting the parking brake cable resolved the exact same problem 12 months ago.

Any thoughts ideas/suggestions would be gratefully appreciated. Thank you!