I have a 1994 Toyota 4runner sr5 with a 6 cyl. 3.0 liter. I have noticed that when I hit my brakes a little harder than a normal braking pressure… not even a hard stop, my rear brakes lock up. My car like many other older model 4runners sags in the rear which also made me notice that the brakes lock up easier when I go over small bumps with my foot on the brake. I am thinking that it might have something to do with the rear springs being weak and not putting enough pressure between the ground rear tires but that wouldn’t explain why it does it even when I am on level ground.
Usually those models have abs on the rear. Is it working (eg ABS light on?)
Also sagging rear may mean worn out shocks. The shocks job is to absorb impacts so your wheel stays on the road not losing contact. Once you lose contact with road the wheel will lock.
Have you checked to be sure the front brakes are working correctly? This includes checking the balance (proportion) of applied force front 2 back. The front brakes should be doing most of the work. Just a reminder not to lose sight of this possibility…
The springs have nothing to do with the rear brakes locking. Brake systems have a component called a proportioning valve that provides balance between the front and rear to prevent the rear wheels from locking. Yours may be malfunctioning.
As already mentioned, make sure the front brakes are working properly.
Also, you have drum brakes in the rear. Drum brakes are “self-actuating”, which basically means that because of the way the shoes pivot to contact the brake drums they tend to press themselves into the drum surfaces. It’s’ possible that due to wear your pads are feeling too much friction on the drum surfaces and combined with tired old return springs they’re binding on the drums.
Start with a good inspection of both the front and the rear brakes and perhaps a rear brake job if needed. Flushing the brake system with new fluid may loosen up the proportioning valve if sticking. If you still have the problem, you might even want to change the valve.
Your SR-5 has a proportioning/combination valve that adjusts the rear brake pressure match the set of the rear axle. If you look under the truck you will see a rod connecting the axle to that valve. If the springs have sagged the proportioning valve reads that as a load in the bed and raises the pressure appropriate to the braking action that would be available. You might be able to adjust the link to give a lower pressure to the rear brakes. The procedure calls for putting pressure gauges in the front and rear lines and adjusting to get the right proportion.
Hope this helps.
Like MountainBike says start with a thorough inspection of the front and rear brakes. Most brake locking problems occur at the brakes and you may be misled about which brake(s) are locking. If the inspection shows nothing, try jacking up the wheels one at a time, and have someone apply the brakes repeatedly with the engine running while you try to spin the wheel. (Use a jackstand as well as the jack unless you are really confident in your ability to keep all portions of your anatomy out from under the car at all times). See if you can get a wheel to lock.
Possible causes include sticky calipers (front), sticky drum brake parts (rear), a chunk of brake shoe material broken off and wedging between the shoe and the drum at times, the proportioning valve, and the brake hoses. The latter are especially suspect on a 16 year old car unless they have been replaced at some point. The inner hose eventually deteriorates and separates from the rigid outer layer. It then allows brake fluid which is under high pressure to flow to the brake, but impedes the reverse flow of brake fluid under much lower pressure when the brake is released.
When you pull the drums you’ll likely find the answer.