Hi, first post, will do an intro shortly.

My new to me 2013 GS350 AWD has a brake light on the dash that only comes on at speed. It never illuminates at a stop. It doesn't appear to have any braking issues and this didn't show up on my initial test drive. The Honda dealer I bought it from has assured me they will fix it but I'm hoping to narrow it down so I don't have to run it back and forth several times.

The only thing I've done so far besides several google searches is added a few ounces of brake fluid though it was a 1/4" above the minimum line.

Welcome aboard!

As you probably already know (by you adding a little brake fluid) when the Brake Light comes on in the instrument cluster it means there is: Low Brake Fluid or a Malfunction In The Brake Booster System (according to the Owners Manual). Obviously it is the latter.

I'm not so sure the Honda dealership where you bought your GS 350 will have the necessary information/instructions, tools and know-how to determine what the problem is and make the needed repairs.

Please let us know the outcome...

Actually, I just added brake fluid this morning and I've only driven the 2.5 miles to work so nothing conclusive yet. It generally was above 50-60MPH where I noticed the light on. SO, still looking to see what happens once I can get it on the road.

Clearly something is amiss that needs attention. Hopefully it's just a sensor that needs cleaning, adjustment or replacing.

Agree with BCL. Something amiss.

My bet is the brake fluid. Reason for this conclusion is we just replaced the master cylinder in our 2016 4Runner because the Brake ABS and Traction Control lights came on. Oh, and the speedometer stopped working. If you had a booster/master cylinder problem you would see more symptoms than just the Brake light I believe.

Yes, I was certain it wasn't anything catastrophic in the making. I've done my share of brake work, including complete replacement on my old cars, and the pedal is firm and it stops well. I haven't had it over 40MPH since I added fluid but it hasn't even flickered so far. Taking a short trip this weekend so that will tell the tale.

Final update. After 60 days of ownership, putting those few ounces of brake fluid in the reservoir has eliminated the problem.

Also check your brake pads, low fluid is an indication your pads may be getting low.

Already did that. Rears are nearly new and fronts are about 1/2.