The 10 lbs valve is used to prevent the plungers from unloading and creating a spongy pedal feel. Be it under the floor or on the stock location. When you mount the master cyl under the floor on a rod is when you need a 2lbs residual valve for the disc brakes, if the MC is up on the firewall on the stock location there is no need for a residual valve for the disc brakes. The drums always need a residual valve of some kind, even when the MC is up on the stock location on the firewall.

I have it setup like that on a couple cars and it works perfect, the Master Cyl is up on the firewall on the stock location, and I have a 10lbs valve for the drums.

Is there anyway of knowing if here is a valve behind the brass seats on a master cylinder without taking out the seats? I dont want to damage the seats but I want to verify if there is a valve in there or no valve at all.-