Charlie, I've had this happen before doing "pressure" bleeding -- where I add about 10 lbs to the brake reservoir. Is that the type of power bleeding you're doing?

A hunk of crud gets in a caliper passageway or even close to the bleeder screw and blocks it. Sometimes I pump up to 20 lbs, after tying a nylon wrap around the reservoir and master - that really makes the reservoir bulge - and the piece makes it through the bleed screw. But sometimes not. That is when the master itself needs to supply the oomph; time to beg for assistance pushing the brake pedal.

I suppose the crud-clog may be the result of a former bad practice- squeezing the pistons back with the bleed screws closed.

With reference to the warning light switch, that last ditch attempt to blast the crud through can set an old switch to one side, lighting the lamp. But in the 240, that is not manually reset by pulling off the electrical contact like it was in the earlier cars. It has springs that are supposed to return (center) the plunger automatically, and the contact pin slides smoothly and does not lock a valve to shut off the bad circuit. But again, crud can hang it up, so blasting through with the pedal may have more consequences than you desired. That's when you go by the book and clean/rebuild/replace calipers and hoses.
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Art Benstein near Baltimore