One of three conditions is responsible for the symptoms your motorcycle is experiencing

1 – a warped front-brake rotor;

2 – a hard spot on the swept surface of on one of the brake rotors;

3 – the transfer of brake-pad material on one of the rotors.

A reasonably accurate way to troubleshoot the problem is to interpret the behavior of the brake lever. If you can feel the lever pulsating back-and-forth while the surging is taking place, the culprit most likely is a warped rotor. The side-to-side movement of the rotor, even though small, causes the pads to move back and forth in unison, and that movement is then translated up to the lever.

But if the lever remains steady while the motorcycle surges, the cause is probably a hard spot or the uneven accumulation of brake-pad material on at least on of the rotors (if you have two discs).

The definitive test of warpage is to check the rotor's runout with a dial indicator firmly attached to the adjacent fork leg. Most motorcycle manufacturers specify a maximum allowable runout of no more than 0.2032mm; anything beyond that would be felt in the lever. The only remedy for a warped rotor is replacement.

Checking for hard spots or brake-pad deposits isn't so cut-and-dried. The best approach is simply to assume that something has affected the swept surfaces and aggressively rough them up with sandpaper. I've generally used 80 or 120 grit, followed by a thorough cleaning with a solvent that doesn't eave any residue.

Most motorcycle manufacturers strongly advise against turning down brake rotors, which already are very thin compared to the automotive style; so, if the sanding doesn't alleviate the pulsing, a new rotor may be the only alternative.