I looked the parts up when replacing my valves to clean out remote oil filter adapter for new engine, so thought I would post it here for everyone to see (with some discussion). Good thing as there was some remaining metal particles and a piece of orange RTV found behind them. (I never used orange RTV during my first build).

According to the factory parts diagrams for a 2001 4WD Blazer 4.3 VinW (L35), there are two bypass valves in the remote oil filter adapter, and one in the engine block. Indeed this is what I found when I went through the original motor.

Per the parts diagrams in the links below:
25014006 (11psi) goes in both the engine block and in the remote oil filter
25013767 (unspecified pressure) goes in the remote oil filter adapter (the one installed in reversed orientation in photo below). This is different than I have seen reported on this forum.
Both valves appear to have identical bodies. Can't remember if the actual valve inside or the spring is different color or not.

Note that these valves open because of oil pressure differentials. In any case I fail to see how a valve opening at 11psi can prevent too high of oil pressure or do anything at all unless there is a restriction that causes a pressure differential above 11psi. As I understand it, ultimately the high end of oil pressure is controlled by the pressure relief valve in the oil pump itself. This is different than some info I have seen on this forum.

These valves are designed to bypass oil if there is a pressure differential like a clogged oil filter or clogged trans cooler. Why it takes three to do the job and the odd orientation of one in the oil filter adapter is a bit hazy in my mind at the moment. Certainly it is not a check valve to prevent oil draining back, as that would require all oil to go through the tiny valve on a normal basis. Anyone have solid info on the way all three bypass valves work together?

I'm thinking that the oil from the engine must go into the remote adapter in the port behind the backwards-installed valve and then passes on through to the oil cooler. After oil cooler the oil returns to the remote adapter and into the area at the base of the oil filter. Then the oil goes through the oil filter and into the center port of the remote adapter and into a passage behind the forwards installed valve on it's way back to the engine.

So if the oil cooler gets plugged, oil can flow through the backwards installed valve and then through the oil filter normally. If the oil filter becomes pluged, oil returning from the cooler can bypass the filter and flow through the forwards installed valve to the engine. If both oil filter and oil cooler become plugged, oil can flow through the backwards-installed valve and then forwards installed valve to return to the engine without going through either the oil cooler or filter. Finally, if hoses somehow become plugged, the valve in the block can open and bypass oil at that point. This is the only way I can figure out how all these valves would work. My remote adapter is reinstalled at this point and I'm not going to take it off to review where the ports go and exactly what is behind them, but I think it makes sense. Your opinions?

What I don't understand is why 2 of the same valves in those particular locations, and a different (different pressure?) one in the other location.

My sources:

http://parts.nalleygmc.com/showAssem...ing=oil cooler

http://parts.nalleygmc.com/showAssem...ing=oil cooler

http://parts.nalleygmc.com/showAssem...ing=oil cooler