ABS?

My guess is the Brake fluid level in the Master Cylinder is not FULL. When Braking hard or accelerating hard at times cause the fluid to move back or forward, causing the float in the master to shift and the ABS/Brake light to go on for a few seconds and then go off again. Have seen this happen may times on cars. They may have did the brake job and bleed it out and forgot to fill the master back up again. Dirt on the sensor or tone ring causing a problem is Rare if that. All that crap that gets all over them besides dirt and medal shavings. About the only way something screws up after working on the brakes is somehow the ABS cable was yanked. I've seen that happen a few times. Normally though I'll just kill it. So I say it's a fluid level problem. I suppose whoever did the brakes ran the brake fluid all out of the system and got Air into the ABS unit you could have a problem. I didn't read anything about a spongy brake peddle though.