On occcasion while driving my 68 Dart, I have experienced a few times
when the brake pedal would go way down. Upon a single pump it would
firm up. I have not got a brake system light during any of these occurances.

On checking the master cylinder I found the rear reservoir to be low.
I also found the fluid to be dirty. After removing the fluid with rags to
soak it up, I found the bottoms of both reservoirs full of sludge. I cleaned
out the sludge and refilled the reserviors with clean fluid. So far the brakes
have worked normally.

Should I expect a master or slave cylinder failure in the near future?

When the pedal goes to the floor, it pushes one or both pistons past their ordinary travel. The part of the bore that is beyond normal travel, over the years, gets gunked up with rust and sludge, and travelling through it is hard on the pistons and seals. So yes, you should start making plans to replace the master cylinder soon. By the book, brake fluid should be completely flushed-through every two years. Almost nobody does this. At a minimum, especially on our older systems not as well sealed as newer systems, the fluid should be completely changed out when you install pads or shoes -- just to keep on top of it and prevent the gum and sludge you encountered. (Remember, "dirty" fluid is also full of water, which greatly reduces its boil resistance.)

I flush my brake fluid twice a year. Keeps the system clean, and its easy to do with the speed bleeder screws (the ones with the one way check valve).

On my 65, and another when I drove it even after replacing the system, new lines, wheel cylinders, and master cylinder I have always experienced a sort of fade. Not everytime but often. Any ideas why, even with brand new parts through and through?

Have you upgraded you wheel cylinders to the new style wheel cylinders?The new cylinders are suppose to prevent air from being drawn back into the wheel cylinders when pedal pressure is relaxed.

Something to think about.

JZ

Tell me more. Can these screws be added to an existing cylinder, or are they only available on a certain brand of cylinder? Where do I get them and how much?

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1970 four-door Dart, 225/A-904/2.45 gears. 0-60 in twenty three minutes!

They are like any other bleeder screw, just with the valve.
Fits "normal" cylinders. Install like the stock ones...

What about the DOT 5 Silicone brake fluid I put in my 1976 Valiant nearly 8 or 10 years ago? I installed all new master and wheel cylinders and new brake lines and hoses so everything was new and clean.

When I take the top off the master cylinder today it is the clear purple color it was the day I installed it and I can still see thruogh the fluid all the way to the bottom of the reservoir and I have never had to add any fluid during this nearly 8-10 year period.

There are a couple problems with silicone fluid.

1) It is virtually impossible to get the air out of it. It traps air in tiny little bubbles that basically never rise to the surface and pop. This makes it very, very difficult to get a nice, firm pedal with silicone fluid. You can get acceptable firmness if you're very methodical and patient about it.

2) It doesn't absorb water. Sounds like a good thing, right? Well...no. Thing is, moisture will enter your brake system. It's going to happen. The seals are far from perfect. The silicone brake fluid doesn't absorb it, but it does trap it. The water migrates to the lowest point(s) in the brake system...and then stays there. Your fluid looks nice 'n' clean way up top at the master cylinder reservoir, but it's a virtual certainty that there is at least one point in the system where there's a heavy concentration of water, doing highly localized corrosion damage.

You may not find out about it until a low bend in one of your brake lines goes "krrrunch!" when you do a panic stop, having rusted from the inside out.

You can combat this with regular flushes, of course, but that mostly wipes out whatever advantage we were using to justify silicone fluid's high cost.

When I was in the army, we got some cars with silicone fluid.
We had to get new brakes for most of them, as the brakes just stopped working!
It was mostly rusty wheel cylinders...

Hahaha, Ive said that before, and wont say it again. Totally agreed: bad idea. I will add this: That water that pools in the low spots not only causes dangerous corrosion, it also boils as the system gets hot and aggravates fade.

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.