[Bender]

Hi psycho. A propotioning valve is just like a tap that restricts the flow. You need to have too much flow/pressure before you can use a proportioning valve to restrict it back. It cannot create flow/pressure it can only restrict it. In some of those early pbr cylinders they had a shuttle valve that operated a switch/light if you had a failure in one system. Its a nylon switch that screws in the side or bottom of the master cyl. Pending on the model. This switch needs to be removed while bleeding the brakes. The shuttle valve will move towards the system that is being bled. ( lowest pressure)(Front or rear) if the switch is still installed while bleeding. 2 things can happen. 1 the shuttle can be held by the switch in one direction resticting the flow. 2 sometimes the switch gets damaged. ie bent. If the master cyl has a switch. I would unscrew it. You may hear and feel a click as you remove it. That will be the shuttle being released.After bleeding the system. Pump the pedal a few times to centralise the shuttle vavle then re install the switch. Hope this helps. If you have more questions, maybe add a photo to make sure its the master cyl I think it is. One other thing I would check is the master cyl bore diameter. I think there was different sizes for the drum brake and 4 wheel disc brake models. This will effect the master cyl to caliper ratio. Regards Bender