If you have a brake pedal that slowly drops towards the floor while you are at a stoplight, then that is your master cylinder leaking somewhere and is unable to maintain pressure to the brakes.
What year/model is your truck? Drum/drum or disc/drum setup?
But to answer your questions based on your photos, Im positive that you have a "Delco Morraine" brake booster and what looks like a disc/drum master cylinder.
Even if you do not find brake fluid leaking from the rear of the master cylinder where it mounts to the brake booster and you haven't noticed any brake fluid on the floor of your garage, does not mean that you don't have a leak.
What I mean is that the master cylinder can develop a situation where it has an "internal leak". In a dual resevoir master cylinder, there are two pistons that displace brake fluid through out the hydraulic brake system on demand. They are directly inline with each other, one for each reservoir. When operating correctly, each piston displaces brake fluid proportionately to their input,(some biasing takes place to prevent premature application of the front brakes and over application of the rear brakes.) The pistons then return to their respective positions once the driver releases the brake pedal. There are restrictive orifices inaide the master cylinder to compensate for each pistons movement.(compensation ports)Any fluid lost is replaced at this time from whichever reservoir corresponds to the leaking part of the system.(front/back half)
Now, if things do not function correctly, due to an external fluid leak or rupturing of the piston seals inside the master cylinder bore for instance, then the fluid displaced by each piston is disproportionate. External fluid leaks would be evident by brake fluid leaks somewhere visible, but internal pressure leaks are identified by the driver having to exert more effort and continually so as to compensate for the pistons seal leaking the pressure needed to effectively apply the brakes.
You posted that the rear reservoirs level has seemed to be lower than the front. If you have a 1/2 ton then the rear reservoir corresponds to the rear brakes, if you have a 3/4 ton then the rear reservoir corresponds to the front brakes (it differs with year too, I think...)
If you don't notice any fluid leaking from the rear of the master cylinder when you separate it from the brake booster, then you have to check the entire length of brake line all the way to the rear wheel cylinders.
I'm guessing your master cylinder is bad. You can perform the tests like the other posts say on the booster to know for sure, but I think your booster might be alright. If it turns out that the master cylinder is bad, then I highly recommend that you replace it with a NEW unit, not a remanufactured unit.(I think they only cost $30...could be different in your area.) And I highly recommend that you replace the rubber brake lines at the same time.(2 for the front and 1 for the rear.)
It'll save you from having to bleed the brake system twice or like in my case 3 times...lol.
Also, you must bench bleed the new master cylinder, it's the only way to purge all of the air out of the master cylinder for certain.
I just replaced my entire brake system over the course of a year. I'm only trying to save someone else the headache and work of doing things twice if possible.
