Homemade pumps all have one problem in common: dead space. It limits the maximum amount of pressure generated by the pump. The dead space is the space between the two check valves and the pump rod.

The washer is used to prevent the pump from going into the check valves, which is good. But the space between the check valves and the washer isn't compressed by the pump, which limits the pressure in the pump. Even if you prime the pump by putting water in that space, it still has the same effect. The water in the pump won't go into the pressure chamber if the pressure in the pump is less than the pressure in the pressure chamber. Since the pump doesn't go all the way to the check valve, the pressure in the pump is severly limited.

Now, what is everyone's thoughts on this matter, and how to fix it?

This isn't a problem at all. Once that space fills up with water, the only disadvantage would be the small amount of unnecessary weight. The water would simply push the other water through the check valve and into the pressure chambers.

More misconceptions are here: pressure isn't generated by the pump. The pump only moves water around in reality. The pressure is created when the pressure chambers have less space for air, and thus the air pressurizes.

The only problem I have with homemade pumps is leaking and finding the right sized O-ring, but once you get correct sized O-rings, neither is a problem. I've also had success using electrical tape as a seal, but admittedly that is a sloppy work-around.

Yes it is, here is an example:

Suppose the water and air in the pressure chamber is at 20 psi. If the pump can't generate 20 psi, the water in the pump won't go through the check valve. Higher pressure moves toward lower pressure. With that dead space, the water can't be compressed completely, which makes less pressure in the pump, which limits the pressure in the pressure chamber. ;)

With high enough pressure in the pcs, water will leak out if you use electrical tape. O-rings are best, matched with the right sized dowel/rod/whatever you use. With too big of a rod, the o-ring won't fit in the pipe. If you don't get the right sizes, you will still experience leaks, but not nearly as much as if you use tape.

Oh, and if you must use tape, use duct tape. Electrical tape can slide right off the pump shaft in one go.

Plus, not everyone uses a washer or other item to stop the rod. I make tracked homemade pumps, so the end of the track stops the pump, which means you can have it go back to within an inch of the check valve and use as much space as you want. It also won't pop out because you can't accidentally draw it out too far.

Pumps use suction to move the water, not pressure!

You still don't understand what I said. Listen to Duxburian and I: Pumps do not generate pressure. Pressure is created when the air is given less space by the water pumped in. The problem would be a combination of personal strength and pump diameter if you could not put more water into the pressure chamber.

Regardless, you bring in more misconceptions. Water is not pressurized in water guns. Water is essentially incompressible for our purposes. Thus, the water is moved around by the pump. I could understand the problem with dead space if we were pumping something compressible such as air because the air will be compressed less with dead space, but water is not being compressed. In that situation, yes, enough pressure to move more air into the pressure chamber may not be possible. However, as I explained, water does not compress and you will never run into this problem.

As for electrical tape seals, that's exactly why I said they are a sloppy work around. ;)

Yes, they use suction to pull the water in. But they use pressure to push the water into the pressure chamber.

I'm refering to the pressure in the pump. It is created when the pump is pushed in. If the pressure in the pump is less than the pressure in the pressure chamber, the pressure in the pressure chamber won't go up because the water in the pump won't go into the pressure chamber.

The pressure will not be less in the pump if the pump is full of water. Water will not compress unless it is put under extreme stress. You've said yourself that fluids have a tendency to not compress (the tendency to decompress). Thus, the water experiences the work normally necessary to compress air a certain pressure, but will not compress because water is near incompressible. The water instead moves into the pressure chamber.

Oh, I see, I guess when I thought of that I forgot that water doesn't compress. If it was air, it would be true.
Maybe my homemade isn't working well because I'm not priming it right. What would be the easiest way to prime the pump?

Though removing the dead space would be good, because then you wouldn't need to prime the pump.

What do you mean by "prime"?

There is no way to remove the dead space completely, and it isn't worth the time anyway.

By prime I mean fill up the dead space with water and still have the pump pushed all the way in. (without pulling the pump)

Just pump it a few times. If you are pumping in air with each pump, your pump is probably leaking.

Actually, there is a way to remove "dead space". Although dead space doesn't really hurt the permformance of your gun, over a while it means more pumps for the same amount of power. What I mean is that if you have a pump volume of 15 cubic inches, and you have a half a cubic inch of dead space, after 30 pumps, you've lost the equivalent of one pump. The water in the dead space is circulating through the system, but is always being replaced with new water.

The only way to truly remove the dead space is by shortening the solvent or thread coupling on the pump side of the check valve. You can remove it completely, and the check valve side of the tee. To attach them together, strong epoxy would work well. You could also use a "sleeve adaptor". I believe they only come in threaded. Be sliding this adpator over the shortened check valvem you would attach the check valve to the tee without creating dead space.


EDIT: "Is moving"? :)