I found the my problem was a lack of carbon deposit on the bottom of the rubber buttons where they contacted the pcb contacts and I struggled to find a solution until a friend offered me some wire glue to try. This came ina 9ml pot and consisted of carbon powder in a non toxic binding agent. It was the consistency of black treacle.
I tiny quantity on a fine paint brush was easily deposited on to the faulty rubber pads. It seemed to dry quite quickly, but to be safe I gave it 24hrs in a warm environment to cure before reassembling my unit.
So far, the buttons work flawlessly, but I can't say yet, how long the fix will last,
Searching around I found the product, Wire Glue, is available from Jaycar, in NZ

http://www.jaycar.co.nz/Service-Aids/Chemical-Aids/Adhesives/Wire-Glue-9ml/p/NM2831

A little pricey at $12.00, but it looks to do the job.