Windmill cylinder condition

Recently I have noticed that a cylinder that seems to pump fine with zero head ( zero column of water) above cylinder) , or pumping from a bucket, may have a near zero capability at 70 or 80 feet. Even with new leathers, this seems to be case. I have concluded that cylinder walls are damaged by grit after a year or two of operation. Therefore, anything deeper than 50 feet, one should seriously consider a new cylinder! With the wall damage, leakage occurs by the leathers, reducing the pumping capacity significantly.