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PostPosted: Tue Nov 08, 2022 3:02 pm 
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Joined: Tue Oct 18, 2011 12:21 pm
Posts: 531
Location: Ontario
I thought that my Bull handpump had a blown-up o-ring after I tried to fill one of my leaking PCP pistol and I decided to take it apart.
My Bull handpump looks like this one.

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I could find several videos showing the sequence for disassembly/reassembly of a Chinese PCP Handpump. Basically, they are fairly good but it is difficult to see the details because the camera is not close enough. Eventually I was able to find some written documentation that had a diagram.

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Finally by laying the components on a large sheet and keeping them grouped in order of stages, it was not too difficult to keep track of where each should go back during reassembly. Unfortunately I forgot to take a picture. Also you should be warned that the tube between stage 2 and 3 has some water in it.

The only part that is trickiest is the Third Stage Piston assembly. I took a picture to be sure to put back the parts in the same order. What I didn’t expect was that the little plastic cylinder going between the o-ring and the brass rod had a flat end and a beveled end which is not apparent from the outside.

Looking for the information about which way it should go back was not easy but one of the sellers (I don’t remember if it is on e-Bay or others) had a picture. From what I saw, it seemed to suggest that the beveled end would go against the o-ring. In my mind it didn’t entirely make sense.

From the tip there is a spring, a metal washer, the o-ring and the plastic piston. All that is pushed against the rod. The rod has a hole that is blocked by the piston. My guess was that the three components moved as a block against the spring when the air pressure is high enough. I am not sure how far the piston could travel but if the flat face is toward the rod, it must move further than the beveled end to let the air go in the rod hole.

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I decided that the person who took the picture for selling didn’t pay attention to the stacking and I guessed that I was right because my way worked.

During disassembly all the o-rings were inspected and I couldn’t find anyone of them that was damaged. I lubricated all the o-rings with silicone grease and reassembled the pump adding new water where was the old one.

When I tested it with the provided plug the buildup of pressure was very quick. In fact I think it was better than when the pump was new.

Anyway, this is another problem solved.

R-Gun Pete


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