thanks Martin just found this

Lscman said:03-03-2004 10:13 AM I don't think anyone has properly addressed the street vs track issues here: Dust boots serve absolutely no purpose until the piston is pushed back into it's bore during pad replacement. As pads wear, fresh piston is exposed and the dirty, exposed portion of the piston does not affect the caliper seal or fluid. A race caliper gets freshened up and possibly rebuilt several times each season. It does not see weathering or high corrosion under race use, so the dust is easily wiped or scraped off as the piston(s) are returned to their bore when installing fresh pads. On the street, the highly corrosive dust that has accumulated and aged on the piston has turned into a solid, rusty crust coating after seeing humidity and rain for months or years. Trying to return the scaly piston into it's bore can result in sealing o-ring leaks. Maintenance intervals and pad wear rates on street setups is usually very different. This is why street setups use dust boots. Dust boots make changing or removing pistons more difficult. Dust boots smolder and/or ignite into useless blobs of rubber at elevated track use temps (over 750 degrees F). Conclusion: If you use race calipers with no dust boots on the street, you'd better remove and thoroughly clean the pistons with a toothbrush before pushing them back in their bores. Race calipers are not designed to have their pistons sticking out of the bores for two years between pad changes & corrosion from being exposed for extended interval & driven thru all weather. The corrosion and crust buildup that forms on the exposed pistons from street use WILL be a problem....unless you're street racing and going thru pads every month. In this case, the frequent wiping will keep the pistons fairly clean. Rebuilding a wilwood caliper typically involves popping the pistons out with compressed air & dropping a $3 viton seal into the groove. This should be done once a year or so. A good mechanic can do this whole job in about 15 minutes, so it should be done every couple times you change pads. The pistons should not need changed more than once every 10 years or so