Sometimes the easiest solution is just to dig in and do a complete overhaul rather than nit pick at things trying to find the cause. Since for the most part this is the off season for riding, remove overhaul or replace calipers and master cylinders and any other valves ( rebuild kits are usually not bad to acquire but clean and inspect the parts you intend to overhaul before you order kits ). Replace the hydraulic lines with stainless lines (you may even want to go the de-link route while your at it), not only does this give you independent front and rear wheel braking but it also eliminates the proportioning valve ). Note: some who de-link their brakes block one port on the proportioning valve, I prefer to remove it completely and connect the line from the rear caliper directly to the master. By biting the bullet and just digging right in and doing thorough overhaul of the brake system you will save yourself a lot of headaches down the road. Skydoc here on the site is a good source of parts for de-linking the brakes as he sells a complete kit which I believe includes cover plates to eliminate the anti-dive valves on the front forks (if do the elimination of the anti-dive valves I suggest replaceing the stock fork springs with progressive springs (if they have not already been replaced by a previous owner ) as this was a very common upgrade on these bikes. All very worth while in doing.