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So you have a Benjamin or Sheridan or Crosman or Daisy pumpup air rifle, but it doesn't pump up anymore. You can have it rebuilt to factory specs, or rebuild it yourself if you buy a seal kit and valve tool. But for a shortterm fix, at little cost and with no special tools, you might start with the alcohol flush.

I heard about the alcohol flush on various airgun forums, and thought I'd try it myself to see if it works. There is probably several ways to do it, it is not a formalized process. This video shows the way that I did it.

And here are a couple of alternate methods set forth in a comment here and by a member of the GTA airgun forum;
Method 1; My method is to squirt, pump 2X, fire, repeat until all the seals hold air and there's no more dirt coming out of the muzzle. No need to soak overnight. Alcohol hardens and dries out the rubber seals so get some oil in the valve as soon as possible after you flush it out with alcohol.

Method 2;
1. Remove the butt stock so alcohol/oil doesn’t soil it.
2. Open the pump arm/lever.
3. Position the gun’s muzzle up & place a tablespoon or two into the tube above the pump cup &
close the pump arm/lever.
4. Place gun (muzzle up) into a bucket (I use a 5gal plastic paint bucket) & leave in this position overnight.
5. Open the pump arm/lever & drain alcohol into the bucket.
6. Cycle the pump arm/lever 4 or 5 times & shoot into a white rag. Do this about 3 times then add
about ½ a tablespoon of secret sauce or pneumatic gun oil above the pump cup? Keep cycling the
gun & shooting it into the white rag until the discharge is whistle clean or your arm falls off,
whichever comes first. If the discharge is still dirty, then repeat the flush. In addition, make sure no
more residues are released onto the white rag, or it will affect pellet flight.
7. Run a few patches through the barrel to clean & dry & you’re done.
8. Be sure to cycle the gun twice before putting it away. This will insure all of your gun’s internals are
protected.
9. Repeat the entire process if no change is detected.