Even though they push through the same shaft, think of them as not being connected. You have two completely separate ways for the brakes to be applied. (I'll lay it out here for others that might come looking.)
1.) Service brakes: When you push the pedal, it lets air in the service brake chambers and pushes on the brakes to apply them.
2.) Spring brakes / emergency brakes: By default (e.g. no air pressure), big springs push on the brakes, applying them. When you push the yellow knob in the truck, you inflate a chamber that pulls these off, so that they are no longer applying the brakes. If a condition occurs where air pressure is lost and drops below about 60PSI (e.g. leak, pushed the pedal too many times, pulled the yellow knob back out), there is not enough air to keep these from applying as so they start to push on the brakes again automatically (by default, until air pressure gets back inside it to remove them).
The spring brakes are pretty powerful, but don't quite generate as much force as the service brake chambers can when the truck is at full pressure. It varies based on the design of the system, but they probably create 60-80% the force of the service brakes.
It's difficult, but there are a couple situations where you could lose all brakes. They make it unlikely on purpose, but nothing is foolproof. If you still have good air pressure in the tanks (e.g. >60PSI), and the yellow knob is pushed, there is enough pressure being supplied to disengage the spring brakes. If you then have an issue that blocks air from getting from your pedal to the service brake chambers, you would have no brakes (unless you pulled out the yellow knob). There are a number of things that could prevent air from getting from the pedal to the service brakes, but they are most likely to occur where you have single points of failure (e.g. it won't happen if you get a crack in one line to one wheel's service brake(s), because air would still be reaching the other 3 wheels and stop the truck), such as a failure in the pedal's valve, the supply line to the pedal valve, etc. Even then, that's why you have both front and back tanks, so that you intentionally have two separate systems that must also fail simultaneously, eliminating most single points of failure. Even the pedal valve is probably two completely separate valves inside, stuck together on the outside (single housing), so that one internal valve can fail and you can still stop the truck.
This is also the danger of caging the rear brakes, and doing anything more than temporarily moving the truck. If anything else goes wrong, you've got zero brakes.
EDIT: Corrected because previously I was saying "pull the yellow knob", but I have reversed it to say "push they yellow knob", to avoid confusion in the future.
