Was the new Master Cylinder bench bled prior to installing it? When bleeding the brakes, are you starting at the right rear, to the left rear, to the right front, and ending at the left front?
Do this. Make sure the Master Cylinder is full and remove the cap. Open the brake bleeder at the right rear wheel and allow it to remain open to see if it will gravity feed. If it does, close it, recheck the Master Cylinder fluid level, and do the same to the right rear, and so on.
If that works, then you can bleed the brakes with a helper.
As far as the bleeding procedures are concerned, read through step 3 in this link:
Here is a link that explains how to bleed the system once the new Master Cylinder is installed:
Let me know if this helps or if you have other questions.
Monday, September 12th, 2022 AT 8:48 PM