I have been using a gatorade bottle and clear tubing, never had any issue. even with freshly rebuilt brakes involving, hard lines, soft lines, calipers and master cylinder (redoing it with new seals).

hook it up to the slave bleeder, keep bottle above hub but below master cylinder, fill master cylinder container, little gentle bumps on the pedal to prime the system and then slow half strokes to push liquid through.

I tried the gravity method with 4 setups. it was slower than mud.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)