Troublesome Triggers

Went out on the tandem today and suddenly found I had only three gears as the right hand thumb shifter decided not to engage, leaving me to change the front ring only.
This gave me very high, medium low and granny gear.
Stopped off at one of EG's old school chums to return a book and scrounged a squirt of GT85 for each shifter.
Seemed to do the trick as the gears came back as the ride progressed.
I would like to change them as they are very old (20 years) but nowadays 7 speed is reserved for cheap bikes and the shifters are bent tin and plastic.
The alternative of upgrading the tandem to 8 or 9 speed is serious money.