Your seals may have gone bad. Do a search on the web for "Air Gun Repair" and locate a shop to have it repaired, OR, See the links below.
Yes you can replace the seals or worn parts. OR, Crosman air gun owns Benjamin. Crosman offers a free service to locate shops who repair old rifles. See the link below.
It depends on the Make and Model of the rifle. If it's a Crosman you can find drawings on their web page (see the link below)However I suggeat that you locate a airgun repair station on the web. Look under Air Gun or Airgun repair to find someone close to you. If you are handy with airguns you might be able to replace the o-rings or seals on the airgun yourself but I suggest that you give it to a repair shop and pay them to do it.
I would start with Beeman air gun co. They own the Marksman line of air guns (See the link below) or go online and look up "air gun repair" and locate someone near you to repair the rifle. Or see the link #2 below.
Made 1978 - 89: Contact Daisy Customer service. They may be able to assist you. (800) 643-3458 See the link below.
lets start with. Who made the rifle? there are over 100 different companies who have made air rifles.
see the link below. I've read that this is one of the hardest air guns to repair.
Your best bet is to look on the web for "air gun repair" and find a location to repair it for you.
It sounds like one of the seals has gone bad. I suggest you do a web search on "air gun repair" and locate a shop near you to replace all the seals. If you are in the USA, See the link below.
you may find that the repair may cost more than the value of the rifle. But you can search the web for "air gun repair" and locate a shop near you to have it repaired or contact customer service at ( Daisy.com ) and ask them.
Mfg 1984: Considering how old this rifle is, I would send it in for repair. Look on-line for "Daisy air gun repair" or "Airgun repair" and send it to a shop near you.
See the link below for a list of places that repair airguns