I believe it is just used to pump grease into the tank, not water. The tank is filled to 100% capacity with water and then the grease gun is used to create pressure by attempting to overfill the tank with the little bit of grease it would take.
I've never done it so I could be very very wrong but I think that's how it works.
That's a good idea, I can just use my existing grease gun without modifying it. I like that and if I am concern with grease contaminating my test subject , I can just use a longer hydraulic hose, let say 18" and fill that with water. Providing the system is well bleed of air and not leaking, I should be able to quickly build up some pressure before much grease flow. My pressure relief valve would need to be close to the grease gun so that after the pressure test is completed I can let the grease ooze out right at the grease gun and not in the test subject.
I don't think the zerk fitting will be a good idea if I was using grease , I would not want to deal with grease oozing out every were. I would do the set up with solid fitting connection. I just thought the quick disconnect feature of the zerk would be nice.
I tried searching the Web and only found one video of AvE
. From that video, it looks like a grease pump can't build pressure with a low viscosity fluid, so a check valve would be needed if water was used. Now, I remember seeing a check valve on that commercial hydrostatic pump unit we had at work.
Just got thinking about the Zerk or grease fitting, they are kind of a crude check valve. Maybe the grease gun don't have internal check valve and they rely on the zirk.