Problem with my FWB 124

Indeed, if the 124 has never been opened up prior it has the original piston seal which by this time in years is totally rotted away and in pieces.

Air rifle headquarters sell a modern material replacement. getting out all the old seal which whats left will be well beaten into the roof and corners of compression chamber needs to be diligently removed.

Common with these 35+ year old treasures and well worth having done by a competent airgun smith or one who has been there before.
 
My FWB 124D , bought in 1979 , serial number 18866. I bought it from Robert Law at the original ARH in West Virginia. I shoot it for many years , but I began to realize that it needed an overhaul. I took it to Steve at Pomona Air Guns ( Baldy Meza , not Pomona) . Steve replaced the piston seal and the spring , then 37 years old. I shoot the rifle regurly , and it is as accurate as ever. Someone who has shot the FWB 124D will marvel how they get over 800 fps from a gun that takes 18 pounds to cock. My HW30S takes that much pull , and for maybe 630 fps. with the same weight pellet. 
 
@ohiochuck, your piston seal probally look worse then this fwb 124d seal.
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I bought my 124 new in the 1980's for $100 because it had sat, seemingly untouched for over eight months, in a rack at a local sporting goods store, and immediately sent it off to Beemans for a complete tune, including metal trigger and all the bells and whistles he offered, Whatever piston seal his shop installed is still good as the gun still shoots Premiers at 750 fps consistently at a mile above sea level.
 
After the tune , you will enjoy one of the finest break barrel sport rifles ever made. Not the most powerful , but one of the easiest to cock for the accuracy and smooth shooting. The Feinwerkbau folks knew what they were doing. I have a walnut stocked , newer model HW35E that is very nice , but second fiddle to the 1979 FWB124D.