I have a Ford Jubilee and replaced the original single acting front lift cylinders with new double acting cylinders with vent plugs in the return ports.This was app. 5 yrs. ago & they were working good until recently.Now, one cylinder intermittently blows the vent cap off & blows oil out (it's always the same cylinder).I was wondering if anybody has an idea what would be causing this?I'm assuming something must be wrong inside the cylinder.Any ideas would be appreciated. Thanks