98 ford windstar brake light

I have a 98 Ford Windstar and my brake warning light has been coming on when I accelerate. It stays on for 4-5 seconds and then goes off. This doesn't happen every time, but happens very frequently throught the day. It started about 2-3 weeks ago. I checked the brake fluid level, and it is fine. Does anyone know what could be causing this, and how do I fix it? Thanks

I'd top up the Master cylinder resovoir with fluid anyhow even thought it appears ok. just don't overfill.

Yes, top the fluid level.

Brian