Indicators stop when brakes applied.

I have an 88/89 35T townmate that had been off the road for approximately 30 years. It has quite good indicators except when the brakes are applied, when they stop all together. Not good or healthy when negotiating junctions! When i acquired it , the flasher relay was missing, and a new one (electronic, rather than electromechanical) was easily obtained and fitted. Also, the indicator bulbs were 21w rather than the 15w ones specified by Haynes. I took a mechanics advice and fitted 8w (6volt of course!) bulbs all round and they were a lot brighter. All earths are good and battery is new and healthy and has over 6v charge, and the brake bulb is the specified 5/21w. Could the problem be that the relay is incorrect somehow? Is there a tried and tested model of relay out there?
Any help here would be greatly appreciated.

Its a normal problem with the T80 that the indicators slow and dim when brake light is operated, battery cant cope, with engine running they should still work. You could try a larger capacity battery.

The answer is a little late, but I faced the same problem, so I made my own flash relay that works at 6 volts, because there was no relay for such a low voltage. So I have a constant time regardless of the load

That's interesting, any chance of a schematic for that 6v relay, or a link to one?

Yes, as soon as i get home to my laptop

Ok, here we are now
Let me start by telling you that I have converted all the lights to LEDs that work at 3 volts. I did this in an attempt to reduce the load from the lighting and charging coils so that I would have an excess of power for the front light but in the end it did not perform as expected.

The basic design is based on the well-known 555 timer integrated circuit.

This is the design, you can see a basic wiring with the 555 integrated circuit, if you don't like the operating time you can change the values of the 47KΩ and 10KΩ resistors or even the 10µF capacitor, the larger the capacitor, the lower the frequency. The S of the small mosfet , the negative of the capacitor and the pin 1 of the 555 must connect to the ground (-) .

This is the old flasher



Here a photo of my own construction



The old flasher has two terminals, the new one has three because it needs a ground to work.

A Brown cable is going to the old signal relay, this is also the input voltage for the new one ,
the output of the signal relay to the signal switch  is the Br/W cable.