I am certainly not disputing what anyone is saying, I'm just trying to get this straight in my head and so far I haven't been able to do it. I going to explain in my mind how I think the metering valve works and if I've got this all screwed up, please point out to me where I'm going wrong.
1) The valve holds off pressure reaching the front brakes until the pressure in the valve reaches 30 to 40 lbs.
2) I would assume there is internal valve of some sort inside the metering valve held close by a spring preventing the fluid from flowing past it. When the fluid pressure reaches the 30 to 40 lbs, that's enough pressure to to overcome the spring tension and the internal valve opens letting the fluid through. (Similar to the way a radiator thermostat opens when the coolant gets hot enough).
3) When you are bleeding your brakes manually (without pressure), you don't develop the required 30 to 40 lbs necessary to open the internal valve so the plunger exists so you can open the internal valve manually.
Right or wrong so far ? Probably wrong!
This is the part I'm having a hard time getting my head around. If the fluid pressure opens the internal valve pushing the plunger out, why would you want to push the plunger back in while you are bleeding like all the books say to? Wouldn't that be closing the valve stopping the fluid flow which is what you don't want to do?
Just trying to get it all figured out......Dave