herman miller swivel chair

Serida

This is a bit of a long shot but maybe someone will know the answer. I have a herman miller swivel chair at my desk (bought second-hand I hasten to say as they are very expensive). There is a height adjustment lever under the seat on the right hand side which works o.k. but if someone heavy sits on the chair it goes down. I wonder if anyone knows how to make it stay where it should be, i.e. is there a way of locking it there? Or maybe it is broken and I shall have to make do with adjusting it each time.
Would be grateful for any replies. Thank you.

Norman_Castle

http://www.hermanmiller.com/contact.html