What might cause the rear axle to lock up under load in a hydraulic brake system?

A defective load proportioning or combination valve can cause the rear axle to lock up under load in a hydraulic brake system by incorrectly distributing the braking force. These valves are designed to adjust the brake pressure sent to the rear brakes based on the vehicle's load, ensuring that the brakes can provide effective stopping power without locking up. If the valve malfunctions, it may either apply too much pressure to the rear brakes or fail to modulate the braking force effectively, leading to a situation where the rear brakes engage excessively or lock up when the brakes are applied, especially under heavy loads.

In this scenario, other factors such as worn brake shoes or a frozen brake caliper could contribute to braking issues, but they primarily affect the brake's performance rather than the load distribution. Worn brake shoes may reduce the braking effectiveness, while a frozen caliper could lead to uneven braking. Excessive weight distribution might cause handling problems, but it doesn't directly cause the axle to lock up unless combined with other issues like a faulty load proportioning valve. Thus, the primary cause related to rear axle locking under load points towards a malfunction in the load proportioning or combination valve.