We had to replace all our reservoirs to use a pressurised brake bleeder as the original tops popped off - link below was the source. I may have a couple of the caps in a spares box but that probaly does not help you if you need all 3. Worrying if all 3 have gone - plastic embrittlement?
https://www.carbuilder.com/products/lockheed-brake-fluid-reservoir
