Brake pressure doesn't always match

[Speedwagon]

2002 Ranger 4x4 XLT.

About 18 months ago, I started to have the rear wheels lock up at low speed light application braking, but only 1 would do it. So I replaced the brake cylinders, as with 220k miles I figured they were due, and they are cheap.

Many months later, the same thing happened again on new brake cylinders. I assumed I just had faulty parts (a bit too common these days), and just replaced them again.

Over the last several months, while I haven't had either rear lock up at light brake pressure slow speed stops, I have had odd issues overall now. I have replaced all the rubber lines, as well as the master cylinder at this point. I also inspected, cleaned, and did a rebuild on the front calipers.

But I get inconsistent brake pressure at the wheels, when not changing the pedal pressure.

Sometimes I will be holding the brake pedal, and suddenly the brakes grab more than they were a second ago. Only feel this effect at slower speeds, so I don't know if it happens at higher speeds.
Sometimes the pedal will suddenly move down more when not changing foot pressure.
Sometimes I hear a small clunk from one side in the front (I don't think it's consistently the same side, but I'm not sure) when applying the brakes at slow speed
I've gotten a C1939, reset it, and had it return.

I'm not sure how to test for the C1939, but I also don't know if that is a symptom or cause here.

How would you suggest I proceed in correcting this issue?

 

[superj]

i wonder if you have a vacuum leak at, or near, the booster?

my ranger will lock the back tires up at slow speeds in the morning. it always has. i guess since its a single cab and is very light back there so the shoes grab quick when cold, since it only does it for the first few miles of driving