The parking brake comes on as in the brakes are dragging, or the light turns on? If it's the read warning light, that is for the parking brake, low fluid level if the van has that feature, (most do), or unequal pressures in the two hydraulic systems caused by a leak.
If the fluid level is low, there is either an external leak that must be addressed, or the front brake pads are worn and a brake system inspection is in order. Never fill the brake fluid reservoir to the top. When new brake pads are installed, if no other service is needed, the pistons in the calipers must be retracted to make room for them and the fluid will be pushed back up to the reservoir. That's why mechanics never top off the brake fluid during other routine service such as oil changes. If you do add brake fluid, be absolutely certain you do not get the slightest hint of grease or any other petroleum product mixed in with the brake fluid.