Brake Lever losing pressure

[curve_in]

I did my longest day so far on my bike - 300 miles.  I had a brake issue come up yesterday.  At a light, I was holding the front brake lever and when I increased the pressure, the lever would slowly move towards the bar.  Any ideas?  K1100RS

[Chaos]

Could be a leak-check you calipers, fittings, hose and most likely master cylinder.  If you're not loosing any fluid from the reservoir, it could be air in the lines.  Good idea to bleed the brakes every couple of years.

[Grim]

  Hopefully its not in the ABS Modulator.

Masters can leak internally. If the fluid level is not changing and you don't find any leaks at the calipers or anywhere else Its probably just time for some fresh seals. As long as the fluid has been regularly changed to prevent corrosion (and you wont know till you take it apart) the cheap fix is a master cylinder rebuild kit. You don't want to know what a Master goes for.

 Now its been my experience that once you get any sort of sign of problem....it gets worse REAL quick. Might be a good idea to park it till you get it taken care of.

[curve_in]

The brakes were done the first of the year so the fluid is fresh.  I got the bike in September and have put 4000 miles in over the winter.  Shortly after I got it, the ABS light started flashing.  I covered the light with tape.  Yesterday was the first time I pulled hard on the lever at a light, so it might have been this way for a while. 

[Grim]

The ABS is bad about going into fault mode with low battery. Thats probably easily reset. There is two version of ABS depending on the Year of your bike. "ABS I" will have Modulators with ABS on them by the pillions foot pegs. ABS II they are mounted next to the battery. Little bit different for each.
This has about the best ABS tech I have found for ABSII.
http://www.bmwmoa.org/forum/showthread.php?16468-K1100-ABS-Problem 
Duck Posted a good one for ABSI that also shows how to make a test harness for reading codes that will work on either version and may work for the EFI as well.

[curve_in]

I'm getting a new master cylinder.  There is corrosion both inside and out plus it has been rebuild before.  I do know that a rebuild would work for now, but I'm really grooving on this bike and I want to just fix things once.  Thanks for the replies!