Before I start the bike in question is a 2013 Fat Bob, 4500km on the clock.

Last week I replaced the hand grips with some new ones. Comming home hte other night I noticed a red light come on on my dash. Checked the codes and found it to be the front brake light sensor had been on for over 2 minutes.

Upon further investigation I have found that my front brake lever now has a small amount of play in it so that when I am doing highway speeds the force of the wind is enough to push the lever back and activate the brake light switch.

Can anyone help me out with where to look to fix this. It has only started since I changed the hand grips so I have obviously done something to the housing, something not seated right?

Happened to my 12 FXDF. There is very little movement needed to activate the brake switch. First make sure your switch block is pushed up as far as it can go towards the master cylinder block. The issue that I found with mine though was that over time the friction between the dust seal at the outboard end of the master cylinder and the piston end that the brake lever pushs on, seems to build enough friction to prevent the piston from returning fully right the way out. Its only a very small amount but enough to give the sufficent slack in your lever which the wind pressure can then push back and activate the switch. Maybe the return spring in the master cylinder is either a) not quite strong enough to overcome any increase in friction due to any sort of crud build up in the dust seal or b) loses a bit of tension over time. Anyways, short of stripping the lot, the best and easiest fix I have found is to periodically give the piston end and dust seal a small squirt with WD40 and work the lever a heap of times, pumpimg up the brake. The piston will then return fully back to the end of its stroke, removing the slack in the lever and light will not come on until brake is used.

Use to drive the guys I ride with nuts when the brake light would come on and off with speed or sometimes the right bump.

Hope it helps

PS - It is also possible to damage the brake switch end when reinstalling the switch block if you are not careful. Pay to check that too.

Thanks for that guys. Well I have just pulled it apart and can find no signs of damage it does appear that the master cylinder piston is very slow to return and doesn't go the full distance leaving about a 2-3mm gap to the lever. I have used WD-40 and worked the lever with only a small amount of improvement.

The brake fluid is a bit darker than I would like so I think I will strip it down and try to stretch the spring a little, clean and bleed with new fluid. Not quite sure why this would happen to a bike with only 4500km on it? Will post results, unless you guys can think of something else to try?

Well I stripped down the master cylinder today and found the oring to be very loose so I got a rebuild kit, $47 from the dealer (small price to pay for functioning front brakes!). See the pic attached for the difference between new and old. Cleaned out a heap of muck and flushed and blead the system with new fluid and it now works like a treat! Thanks for your input guys.

2013 bike,I would have thought it would have been covered under warranty...

+1. WTF, Something fishy here. 2nd hand components put on by mistake? Wrong brake fluid perhaps.  I would seriously question the dealer on this one. and get ya money back on the kit.

Well its back. Brakes have been functioning as they should until coming home today and noticed the warning light on again. So far I have put a master cylinder kit in and have bleed new brake fluid, not sure what to do next? Any suggestions? Not all that keen to take it to a shop and throw heaps of $$$ at such a small problem.

Anyone got thoughts on stretching the cylinder spring? I measured the old one and its 53mm, might try to stretch it 3 or 4mm and give it a try.

Let's get back to basics shall we. You stated that you changed hand grips and then the problem started right?
Well its is something you have done.
There is a little piece of steel bent back on itself that holds the brake switch in place. Have you double checked to make sure that is there before stretching springs and stuff.
I have never heard of anyone doing that before. It will be a simple problem. You just need to find it. Don't over complicate things as it will end up costing big moolah.
Hope this helps.

Thanks FLHuTChU, Im with you there! I have checked all and it appears the master cylinder is not returning fully. Took the lever off and I can pull the piston out with a pair of plyers to where it should be. So I am assuming the problem is in the master cylinder. I made a small shim out of a disc of plastic to fill the gap and it has temporarily fixed the problem. Will see how that goes before I start stretching springs.

On my bikes the front brake light is activated by a small switch that pops out when you pull the lever and is pressed back when you release the lever and has nothing to do with the actual workings of the master cylinder...Often what happens when people work on there grips or levers that little switch gets broken off and the brake light stays on...where have you put the shim??? whoops a daisy...I just re-read your original post and your talking about a warning light on your dash and not your brake light....I have no idea...

Thanks Sparra. The warning light comes on after the brake light has been on for over two minutes. My brake light is on because the master cylinder isn't returning to its full extent to release the switch, same as your set up. I have placed a small shim of plastic, about 1mm, between the master cylinder piston and the lever which has made up the gap. Have only tested in the garage but all the play has gone, will test on the highway tomorrow.

I had the same problem, front brake causing brake light to come on when hitting a bump or staying on after use on my 2014 fatbob. I took the brake lever off and put a couple of pieces of tape over the area that pushes on the brake switch. This just means the brake lever has to move a little further before the switch actuates... Might not be the best but it works.