I was on the AC Delco sight looking for brake parts and came across some cool information about the brake proportioning valve...actually they call it a combination valve. The valve consists of : 1) Proportioning valve that controls the front to rear brake bias. 2) Pressure differential switch that senses pressure drop between the front and rear which activates the brake waring light. 3) Metering valve that delays hydraulic fluid going to the front brakes until 30psi. "The metering valve is used on rear-wheel drive vehicles with front disc/rear drum brakes. Disc brakes react quicker to intial brake application than drums due to tighter pad to rotor clearances. The meteing valve delays front brake application for a fraction of a second so the rear brakes engage at the same time as the fronts. In some instances, the meteing valve can prevent fluid flow to the front wheels during pressure bleeding. Therefore the valve may have to be disabled when pressure bleeding. Special tools are available to disable the valve by depressing the valve stem. From 4 to 30 psi, the metering valve remains sealed, closing off brake fluid from the front brakes. The master cylinder delivers brake pressure to the rear brakes only. Hydraulic pressure acts against the drum brake return springs to bring the brake shoes in contact with the drums. Avove 30 psi, the hydraulic pressure begins to overcome valve spring pressure, opening the metering valve seal." (paraphrased from AC Delco) So when the rear pads contact the drums, the hydraulic pressure raises quickly which overcomes the metering valve and lets fluid flow to the fronts. Since there is no air in the system and the fluid doesn't compress, it's pretty much instant. I thought this was good information because it actually explains why some manuals say to depress the metering valve stem and others don't mention it. It seems to me that if you bleed your rear brakes first and have them adjusted properly, you would not need to depress the metering valve stem. But if you just bleed the rears, and have not adjusted them yet, you probably would need to depress the metering valve stem.