Bleed

The proportioning valve may be the issue.

This multi-function hydraulic control unit consists primarily of a pressure differential valve, brake lamp warning switch and a proportioning valve. A pressure differential shuttle and/or metering valve are also incorporated with some valve applications.
The metering valve delays front disc brake effectiveness until the rear brake shoes contact the brake drums. The proportioning valve regulates rear hydraulic system pressure to prevent rear brake lock-up. The pressure differential valve senses hydraulic system pressure and will activate the brake lamp warning switch if a pressure loss occurs. The pressure differential shuttle bypass feature provides full rear brake application if front hydraulic system pressure is significantly reduced.

Roy