Gord is correct, but I will take that one step further.
There are two terms of "bypass" used. One as Gord stated. The other is a pressure protection mechanism. Depending on the engine design, the "relief valve" can be found in either the engine, or it maybe found in the filter it'self. The Oil pumps are a "positive displacement
device". Meaning that every rotation, the pump delivers a set measure of oil. The fast the rotation, the more oil pumped. This means that if the flow is restricted in anyway somewhere down stream of the pump, the pressure will keep building till something blows or breaks. The filter is the usuall place of restriction, due to blockage. If the pressure continues to climb due to the filter getting blocked, it will either rupture the case, or worse, it will fail internally allowing sludge and paper to flow through the engine and possible damaging it. So a relief valve is fitted. Once the filter restricts the flow to much, it overcomes the valve and and the oil bypass the filter and flows through the engine. It is better to have unfiltered oil than no oil at all. Now, this relief or bypass valve can be found in some filters, or it can be found in the engine it'self. If there is no engine fitted valve, and a filter with full flow through is fitted, low fluctuating oil pressure may result.
There is one other valve as well. this is the pressure valve that sets the actual working pressure of the pump. As I said above, the pump is positive displacment. So it pumps a set amount of oil every rotation. At a slow idle, this displaced flow my not be a lot to keep up with all the galleries in teh engien, so you get a low pressure reading. When you rev the engien, more oil is pumped. The galleries have to be able to get rid of the flow or seals
will be blown out and damage to the pump may result. So the pressure relief valve allows the over flow to bypass back to the sump. It is this valve that sets the upper pressure limit. In some more simple designs, this valve can do both jobs. But I imagine the later model perkins
you have would be a litte more sophisitcated than that.
Sorry, I can not help with what scenario you have. I have not been inside one of these engines nor seen a manual.