1961 Dodge Custom no power on brakes

Hello,
I'm writing from Germany and here I have a 1955 Dodge custom royal in a very good condition. The year in the required field is wrong, must be 1955 but isn't available, so I hope you can help me, too.
The repair-shop replaced on the front axle to disc-brakes with all needed components. On rear-axle remains the old drum-brakes. But we didn't replace the mounted brake-booster, but installed a new 2 circuits-master-cylinder 1" bore-size and a proportioning-valve (no combination valve) plus a residual-valve for the rear-axle.
In fact, the system doesn't work well, because there is no sufficient brake-pressure. For a better location of the problem the mechanic blocked the brake-lines on rear-side. In this case the pressure on the disc seems ok. As well turned around with blocking the lines to the front. But not together. Now the mechanic mentioned to me to replace MC + booster + prop. Valve to a assembled kit which is offered by many supplier from Internet with the advantage of good balanced parts. As we (mechanic and me, customer) are having no experience to chose a proper kit, I'd like to ask you for a recommendation what kit do we need, mc with x" bore-size, pedal-system, etc. Or may be you have a brand for a kit. All is well-come. On the other hand we would like to understand what went wrong.

Thanks for a nice replay. If you will need more information don't hesitate requiring pics and others.

Best regards,
Jörg