My Derwent Macdee 9,5" flush system flow is low?

I checked the membrane and appears Ok. The flow is very low. Actually there is no "flush". The mechanism is so simple that I would accept that this models do not flush but let a small quantity to flow for some seconds. I could replace it but does anybody know if this is the way they work? I would not like to have the same results. Thanks

John Papas
October 2010