Mercedes Benz repair: 1980 Mercedes Benz 240D, mercedes benz 240d, mercedes benz

Question
My parents have a 1980 Mercedes Benz 240D W123 series with about 150 000kms on the clock.  When the car is cold it takes about 2 minutes to engage reverse gear.  The engine is at normal revv, box has never been opened, the oil is clear.

Is there a filter?  or, is there a bigger problem?  The problem has started gradually.  Any suggestions?

Answer
first off I would change the transmission filter and filter in the gearbox then I would look at a transmission spring replacement for the reverse gear and also a new modulator
http://www.benzworld.org/forums/w201-190-class/1361737-diy-mercedes-722-3-722-4-...
here is a good idea of the replacement you will have to call up the dealer and ask which spring controls the reverse on your transmission