Really tricky to diagnose electrical. All contacts, wires and relays should be inspected. It still is possible that 8.5v that you measure is too much for your 8.5v glows, they are exposed to the max they can take for 20 secs every time you glow. Which maybe why they burn out.
It is an old car, who knows if a pvo did something, copper corrodes, a superglow system is not as fail safe as a wilson switch.
For example, my superglow ran on 6v glows from 1988. I pulled out my superglow relay (electrical relay is over 30yrs, chances it is not tip top) and bypassed the relay with a wilson switch. Installed 11.5v glows to avoid burning them out. When I hold the button down, I watch the $3
ebay cig lighter voltmeter. It drops from 12.6-7v to 11v (12.5v to 10.4v if the battery is not healthy) as the glows drink and warm. I count to about 16, sometimes more during winter less during summer whilst I watch the volts. The volts climb back to 11.3v (little less if battery is old) which is a sign to me that the glows have stopped drinking and are warm, I kick over, starts every time. The glows are several years old, the last time I changed them, the old ones looked close to unused.
Using higher voltage glows takes me 12 seconds longer-ish than a superglow. But as I am watching the voltage I am ensuring that I do not cook my glows, by kicking over exactly when they are warm. It is fail safe.
Further more, I use an engine guard temp gauge , a thermo couple which bolts to the head. It is really accurate. After I have already run the engine, driven wherever and switched off, naturally the engine cools down. A glance at that reading tells me if I need to glow again if below, say, 40c-ish.
So the combination of reading the volts, counting the time and monitoring the temp , I act as the superglow.
There are two very simple computers in your car. The superglow and the edic relay. It is most common that they both fail over 30 + years, which is actually pretty good for electricals.
The older b engine had a little coil which heated up and glowed in cab as one manually glowed by counting and watching the coil.
I know a young fella here in melbourne, who is making a replacement for the superglow, it shall have an inbuilt wilson switch, if ever, needed. He is already making and supplying edic control boards already and can be found on facebook, he is on ih8mud somewhere too. The 3b and 2h have the same electrical setup virtually.
I'll try to get him to chime in, seeming you have the 12v system. He is working towards 24v systems too.
