'97 Chevy truck brake help

[R Q]

I'm working on my '97 Chevy truck front brakes. Replacing calipers, and flex lines. Before doing all that I am bleeding all of the old dirty fluid out.
The master cylinder has two chambers, front and rear. I thought that the front brakes used the rear larger chamber in the MC. Well when I'm bleeding the fronts, the small front chamber is draining. Then I change to the left side of the truck (still front brakes) and both chambers start draining. I was keeping the fluid below the slit between the two chambers so they wouldn't mix and to keep my fluid clean and I never let it drain down too low or run out.
I still have the back brakes to go (Bleed) and will continue on here as soon as I post this.
Does the fluids draining from the separate chambers mean that I have a bad master cylinder or proportioning valve?
The reason I'm working on them is that I thought I had a bad/stuck caliper which has happened a few times before. I'm also replacing the rubber lines because I've learned that they can collapse and/or swell and hold pressure creating the one side drag. They also are deteriorating and putting black gunk in my fliud, causing even more problems.
Thanks,.
RQ

 

[Blkvoodoo]

Odds are if the fluid is that nasty, and you believe there are issues with calipers and hoses, the master cyl is just as effected. for the cost it's an easy swap out.

I would also suggest before you get too crazy with bleeding the rears/replacing wheel cylinders, that you also check the hard line running under the bed on the frame. those trucks are very prone to rusting out that line due to the spiral wire wrap around the tube holding water. if you are bleeding the system and flushing out the old fluid, at least check this, if not replace it, the increased pressures of the new fluid/system will find the weak spot fairly quickly.

As for the murky brake fluid, much of the murk is from deteriorating rubber seals and such, but a lot can be attributed to the moisture that brake fluid absorbs over time ( brake fluid is a type of alcohol (polyethylene glycol)). The moisture in the fluid will heat up and cause boiling of the fluid, which can cause a very spongy pedal or in some cases the expansion will be enough to cause a caliper to drag.

 

[R Q]

All duly noted there Blkvoodoo. Thank you. I did the hoses and calipers and bled it all good. I think the MC is next. The pedal just doesn't feel right. I had the booster changed a short while back, surely he would have replaced the MC as well? I'm gonna look it up and see.
I was thinking that I need to replace rear cylinders next so thanks on the heads up on the line under the bed.
I had the work that I did today priced out at a shop that is supposedly good. For both front rubber lines and calipers and bleeding the whole system he wanted $506.00
I have roughly $100 in parts max and maybe 5 hours and I ran and got my own parts (and lunch) and I don't do this stuff every day.
Edit: MC was replaced this spring with the booster.