Every time I want to clean a sac converter (Parker, Sheaffer...) for changing to another ink, I'm a bit paranoid: could I break/ruin it by repeatedly pushing and releasing in order to flush until clean? And then, how to be sure it's dry inside before filling again, as I don't want to dilute the new ink?

How do you clean/flush it?

I flush it same as I do with piston converters, absorbing and releasing clean water until no more ink; but because of the sac being rubber and because of its age (both are 70's converters...) I'm really paranoid about reducing its life by doing that way...

Are there any other cleaning methods that I should do instead?

Or, do you recommend to change it for a modern piston converter? If so, which model would work in my Sheaffer Targa - standard, NOT slim?

Thanks!