If you think using a liquid gauge is not beneficial, just try using a gauge without liquid and let us know what you find. From my understanding the gauge will break, but that's not what this thread is about. There's a guy who has been very helpful that has very good YouTube videos on pop testing and cleaning injectors for the OM617. He told me that my pressure seems to continue to rise even after I've stopped pumping, and he thinks it's a result of going from 1/4" fittings up to 3/8" fittings. I ordered the right fittings and will hopefully have an update tomorrow.

The gauge does seem to be delayed because from what I can see the injectors begin chattering at 400 PSI. Hopefully this is due to the fitting sizes and not a faulty gauge.

Fitting size will have absolutely no effect on pressure. Liquids are non-compressible so the pressure will be the same regardless the line (or fitting) size. If you check the orifice size on the gauge it is going to be a lot smaller than 1/4". If it were to have an effect, you would need to order a gauge with the inlet size the same as the line size (or tank size, if you were checking the pressure of a tank).

Line size does have an effect on flow rates, but I can assure you that the orifice on the nozzle is much, much smaller than the line size going to the injector.

I have been using injector testers with dry gauges for 40 years. Haven't broken one yet. And I have never encountered a manually-operated, professional tester (Bosch, Bacharach, OTC) with a liquid-filled gauge. Nor can I imagine a single characteristic of a manual tester that demands a liquid-filled gauge.

Why would you prefer a slow-to-react gauge on a pop tester?

Who said anything about "demands". I used a liquid filled gauge on my home-made, DIY, el-cheapo bottle-jack-based, perfectly serviceable pop tester because that was what was available and within my budget. I don't need a multi-thousand dollar unit for the once-every-year-or-so-pop-test of my injectors. I'll put the money saved into tools, fuel, gauges or pop corn.

My scale goes way too high too, but for me and the design criteria of the unit I built ...

... it works and it was within budget

Any ideas on why the gauge slowly rises and then ends up falling? I do not see any leaks by the way.

you could have some fluid leaking back into the pump assembly

How might that happen?

Have you ever used a floor jack and had it leak down when the car was on the jack? could also have some cavitation that is settling out but i think that is less likely, also part of the load does go back through the return lines, is there any diesel showing up there? i think it moves that fuel right as injector closes.

Disclaimer: I did not write it
 

Liquid Filled vs. Traditional Gauges

The primary reason for gauge failure is vibration. In traditional dry gauges, delicate links, pivots, and pinions are prone to damage from vibration, leading to inconsistent performance. In a liquid-filled gauge, the fluid fill effectively dampens system pulsation, making the gauge pointer easier to read.

Likewise, humidity and moisture pose problems for dry gauges. Condensation can make a gauge difficult to read or, in some cases, can cause failure due to icing. Liquid-filled gauges, though, are sealed to prevent internal condensation, and by keeping moisture out of the system, gauge movement is protected during icing conditions.

The choice of fill fluid is based on the application, but glycerin is typically used for room-temperature applications and silicone oil is commonly used for extreme temperature applications, especially when icing is a problem. An oil with insulating properties is ideal if the system has exposed electrical contacts.