Confession: I never (never in 40+ years!) thought about pushing the piston to expell old fluid. No wonder me and my Mity-Vac pressurizer could get thru a flush on less than a quart! Never too old to learn is right.

Question: Why clamp the hose? I can't see how any appreciable amount of old caliper fluid is going upstream into ABS territory, given that:

1. You've got a bleeder open (path of least resistance)
2. The piston pushing is (or could intenionally be) relatively slow.
3. The amount of fluid that tried to "the wrong way" would be almost negligible, compared to what comes out an open bleeder. And not worth possibly damaging a hose. It just doesn't seem right.

OK, its learning time again...
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Bruce Young, 940-NA, 240s galore (one V8 just sold), '73 142, '71 144 (ex circle track car in CT), '67 144 (new), 122 '63 to '67