ABS service light cycles on/off?

2000 Astro RWD
The brake warning light keeps coming on periodically, sometimes with the first use of the brakes, sometimes after driving a bit longer. And then it goes off when the van is shut down, and upon starting again no light. Then on again, etc.
One mechanic couldn't find a brake code in the system error file, but suggested improper bleeding or lack of bleeding could be a possibility.
Any ideas?
The brakes work fine, the front was redone 10,000 m. ago and I keep checking the fluid level; it's fine.

Is it the red brake warning lamp or the amber ABS warning lamp? Does the amber ABS lamp cycle on when the ignition is first turned on?

Both the red and amber lights come on at the same time, but not when the ignition is turned on. They're off for varying amounts of time and seem to come on with brake usage, but not necessarily instantly when I use the brakes. In other words, if I hit the freeway and don't use the brakes much, they stay off; if I'm driving in town and have to brake a lot, they'll come on sooner. But not right away when I start up.