Y, I think brake fluid will do it, DOT3 is best, DOT4 can damage most plastics... However, Castrol Super Clean is better. Many people use this stuff on plastic models and miniature figures when they want to strip and repaint them.
1. Wear nitrile rubber gloves. Important, this stuff will burn your skin. If you are using brake fluid, it can be absorbed through the skin and is toxic....
2. Soak the caps in it for a while (leave it long enough to soften the paint all the way through, overnight is probably a good choice).
3. Use a toothbrush to rub off the paint.
4. Wash with warm soapy water and rinse. Job done.