I have been going through everything on our 57 Isetta that we inherited from father and its now up and running...But when I went to open the brake master cylinder the cap broke leaving part of it still in the master cylinder.  I got that part out of the master cylinder. 
Any ideas were I can get another Master Cylinder Top Filler Cap or find a substitute??  
PS.  After I changed all the fluids it start right up after sitting for 10+ years!!  My father did a heck of job restoring this!!