Improperly adjusted, new pads, low traction, etc.... most vans only had rear-wheel antilock brakes.
Check the pistons inside the calipers, usually they are the reason brakes lock up.
When you hit the bump and you have the brakes applied, the tire will lock up when the tire is air-born for a moment.
Disc brakes in the front, drum brakes in the rear. Anti-lock brakes were optional.
Rear drum brakes, that lock up during heartbreaking quicker than the front brakes, can be adjusted. The rear brake pads need to be backed off.
Slam on your brakes. If your brakes lock up, then you do not have ABS. If your brakes do not lock up and you hear a knocking type of noise from your brakes, then you have ABS.
remove them greese them up if that dont work replace seals or buy new ones
old break hose collapsing and or pistions sticking in calipers.
Bad fluid in master cylinder,flush system,bleed brakes
Most likely you do not have anti-lock brakes in your vehicle. Other possible answer is that your anti-lock brakes are malfunctioning. If you have anti-lock brakes you should see a mechanic as soon as possible.
The Anti-Lock brakes pulsate so the brakes do not lock up (anti) and cause the vehicle to side out.
Be sure to change both front brake hoses. I didn't and my calipers would lock up.
in many cases, contaminated brake fluid is the problem.