Reminds me of my F2 other than "locking part".Head seemed to raise ok and all but was slow on variable.Backup rubber in pump failed and blew seal out shaft side.Put O_H kit in pump and then variable worked like a champ along with all else.Rather cheap to rebuild pump and well worth the effort.You can bet yours is weak.
That was 4 yrs ago but it seems like O_H kit was cheap.less than $100 ,more like 30-40 I could be wromg but it was easy to install and made such an improvement.Yuo'll be glad you did it.