I have a '71 2800CS that has 3.0CS brakes (vented discs front and rear with rear 3.0 semi-trailing arms). Although the hydraulic system doesn’t leak since I rebuilt the calipers, I have poor balance with much less braking in the rear than expected. I suspect that either the proportioning valve is defective or the master cylinder is either defective or incorrect. I don't know anything about the replacement history of the current master cylinder.

Can the master cylinder be identified by external characteristics? Will using a 2800CS disc/drum brake master cylinder with the 4-wheel disc brake system cause this brake imbalance?

Could this problem be caused by a defective proportioning valve? Can the brake proportioning valve be cleaned or rebuilt?