There are three nuts, that hold in place, a triangular brass plate, on the right side, (facing from the front), of the rear boiler. On my early GS/3, SN 093, all three of these nuts were loose causing water to leak into the brain box.
A quick tightening of these nuts, with a 13mm wrench, could save your brain box. I found the problem too late to save mine.

A quick test revealed that several other boiler bolts were loose. Try tightening all of them.
The brass triangular plate, on the front boiler, is held in place by six nuts, two to each bolt. Doubling-up the nuts, on the rear boiler brass plates, would be a good idea as well.