My bike is a 2002 bmw r1150rt with the abs warning lights flashing. i have had the brake fluid service done but it is still their, it is fine when cold but after 15 to 20 miles the lights come on again

Low voltage has been blamed for ABS faults on a number of bikes, and those bikes have been fixed by going to a different battery, usually a sealed one. Sealed batteries have significantly higher voltage due to the chemical differences, usually about 12.85 to 12.95, instead of the 12.60 to 12.80 typical of flooded designs
If the lights initially flash together but alternate after you start the bike, you can try the "rolling restart." Ride away with lights flashing and on a straight section of road, in third gear or higher and maybe 2500 RPM, pull the clutch in and turn the key off with your right hand. Coast for one or two seconds, then turn the key back on and release the clutch to "bump-start" the motor. If the lights are flashing together, you're in luck, and next time you come to a stop and pull away you should get a successful ABS self-test. If so, you need a new battery or you have some other charging system problem. If not, you have some other fault.
There is also a modification you can make so that the ABS unit is not powered up until after the motor is running; this will also cure low-battery problems.
BMW batteries often (in my experience) have quite low voltage. This might be a carry-over from the days of the airheads, where cranking current was vital, but voltage wasn't. As you go to higher voltages, the cranking performance can suffer a bit, especially at low temperatures. More importantly, they are harder to recharge - an important point for most airheads. I have seen Mareg (BMW) batteries on properly functioning bikes at 12.40 volts and lower, fully charged, engine off - well below what is considered (I think by BMW) to be the minimum for good ABS operation.
Dealers get the electrolyte for the batteries locally - not from BMW - so this aspect of the battery is not really BMW-controlled. Different dealers could be using different strengths of electrolyte. The article explains how this affects voltage. If your fully charged battery is at too low a voltage due to weak acid being used to fill it, you MAY be able to successfully raise the voltage by use of fresh electrolyte.
Theory only! A way to do this would be to fully charge the battery, discharge it to a point where the electrolyte specific gravity is about .030 below the new electrolyte you have, then empty it and quickly refill with the new stuff. This will give you maybe a .020 increase in gravity and a 0.2 volt improvement. Don't shoot for anything higher than 1.285 or so. Remember, for a 12V battery the voltage is roughly ten times the acid gravity. 1.260 S.G. gives you around 12.60V. And don't let the battery stand without acid in it.
If your problem is that the voltage drops too much when the key goes on, then the above IS NOT your problem. You probably want a stronger, or newer, battery. The success of the new sealed batteries in this area is largely due to the fact that the voltage doesn't drop as much while cranking, rather than simply higher overall voltage. This is a function of plate area, etc...

hope this helps

John