I also had this problem with a 450SL that I had for 19 years....only it was even slower to engage, took about 2 seconds. I bought the car used in '87 and the previous owner said he was told that it could be fixed but wouldn't hurt the transmission to drive it the way it was. I guess he was right...the tranny was still doing fine when I sold the car a year ago after putting about 50K miles on it. If my '86 follows this pattern looks like I still have many years and miles before reverse stops working.

__________________
1986 560 SEL (159K miles)