I agree with 97 SL320. I had the same issue with installing the Centric boots (silver/silicone type) on my 400E upgraded calipers. I was worried that I would not be able to use the brakes without fluid running out. Went to my mechanic after destroying 2 sets of boots and asked him if there was a trick and he said use silicone to lube the rim and it has to be pressed on squarely either by hand (near impossible) or with a tool. The boot seemed rather thin and I didn't want to risk destroying a third set. He advised using OE boots or reusing what I had. I told him my concerns about fluid pouring out of the boot if it wasn't seated properly or had a small hole, which mine did. I said the boot did not prevent the fluid from escaping the caliper, the inner ring did. The boot was just there to keep dirt out. He said I could even find some rubber or silicone sleeve that fit over the piston and use that as long as it could withstand the heat of the brakes. I reused my other boots and have not had any issues with the calipers since. Hope this helps.

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1990 190E 3.0L