Thanks for your suggestions! Yes, I have tried it with and without a proportioning valve. Doesn’t make any difference. The brake lines are clear, and all of them are hard lines (steel tubing) except the flex to the front calipers (new) and to the rear axle (10 years old at the max). You can see why I am so frustrated.
When you step on the brakes, do the front brakes go on first or do they just apply more pressure? I’m still wondering is there could be a mismatch between the booster and the MC.