Removed right bulkhead cap and epoxied map light nut in place. Reinstalled bulkhead cap and began blind riveting. Bad problems with LP4-3 pulled rivets breaking long; about 1 in 4 ended up breaking at the tail and were very difficult to complete. Had to remove nose piece from puller tool; one rivet shank failed and pulled out of the hole; hole was ok. Made steel mandrel spacer to replace nose piece of puller, allowing the tool to regrip the mandrel. Required extreme effort to rebreak remaining rivet mandrels. Suspect bad rivets; further investigation is warranted.