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Interesting. Yeah it definitely came out the cup. After that I was initially able to feed liquid from the calliper to the lever. At the start there were loads of micro bubbles, frothy almost. Then it got better.
I bled (or tried to) as per the shimano R8070 dealers manual. Fill from the bottom, gravity bleed to remove bubbles, depress lever and open/close valve on the calliper quickly. Oil in cup and pump lever, moving bike angle and tapping everything to get bubbles out.
That wasn’t doing much so I started again and no oil would pass through the system.
I have tried loads since and it looks like I can’t even gravity fill/bleed with a cup at the top. I even left it overnight with a full cup and the hose disconnected at the calliper. Nothing left the cup. But if you pull the lever, oil comes out the hose at the bottom.
cjr-
Fluid can only flow between the reservoir/cup and the lever cylinder when the lever is in the fully relaxed position. If you push the lever away from the bar does the "blockage" disappear?
Diagram here. The bleed cup hole is on top of the reservoir:
https://en.wikipedia.org/wiki/Hydraulic_brake#/media/File:Hydraulic_disc_brake_diagram.gif
grams
When oil blew out of the bleed cup, did you see it come out of the cup? It sounds like the over pressure valve.
Just to check, are you doing the bleed as per Shimano’s instructions? It’s late and I might be mistaken but I thibk the procedure if: bleeding bottom-up until no more bubbles, remove bleed cup, seal shifter, pull brake lever all the way and hold to the bar, gently repressurise with syringe while slowly allowing brake lever to release, do not add excess pressure with the syringe it should move the lever all the way out and feel firm and that’s all, seal calliper, test.