BRAKES LOCK UP WHILE DRIVING 1989 NISSAN D21
I am interpreting your description to mean that the brakes seem to work fine for a while, but then they fail to release after a period of driving. If sounds right, the most common cause for this is that one or more of your disk brake calipers is sticking, failing to release even after you let off the brake pedal. Normally, only one caliper will fail at one time, meaning that the vehicle will pull strongly to the side where the brake has locked up. You can also test this theory by driving your Nissan until it starts to drag. Pull over and lightly touch each of the tire rims. One or more of them should be hot--possibly burning hot--to the touch from all the heat that's been generated by the stuck brake caliper. A wheel with a properly working brake will probably feel warm to the touch--but not super-hot, unless you've just completed a motorcross course or something that gave your brakes a real workout.
You can buy rebuild kits for most brake calipers, but, in all honesty, it may cost almost the same and be better in the long run to buy a factory-rebuilt caliper, trading in your old one for its core. Factory rebuilt calipers are sandblasted and carefully cleaned and painted before the new parts are installed; quite frankly, I've never done any of that when I've rebuilt my own calipers.
It's possible that you might have frozen calipers even if both brakes on an axle (i.e., both front or both back wheels) are locking up. You might have gotten moisture into both of them by driving through a deep puddle, etc.--or you might not have noticed the problem until both calipers started acting up, holding back your car even more. I don't believe that your truck has any kind of special one-way pressure relief valves in the brake lines, outside of what's in your master cylinder.
Another cause for your braking problem might be your master cylinder. Ordinarily, master cylinder problems show up as loss of braking--your brake pedal goes to the floor or becomes very mushy. However, it's possible that a faulty master cylinder with some internal corrosion could be failing to release pressure after it's been applied. If you've excluded the brake calipers as a cause, this would be the next place I'd check.
A final thought is to think about the age of the brake fluid in your system. If it's older than a few years--or you can't remember the last time your system was flushed--chances are that the fluid is old and has absorbed too much water to perform at its original specifications. I don't think that water in the lines would tend to lock up your brakes, but you could try flushing out your brake system by bleeding all the lines and feeding a steady supply of fresh fluid into your master cylinder. Start with the rear brakes and bleed the front brake that's closest to the master cylinder last. This won't be a quick pump the brakes a few times and look for air bubbles kind of bleed. Instead, you'll want to pull out a half cup or more of fluid at each brake (more at the back, to compensate for all the fluid in the extended brake lines), ensuring that fresh brake fluid is being pulled into the brake lines and into the brake fluid reservoirs at the calipers and at the rear drums.
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