When should a trailer brake master cylinder be rejected?

The rejection of a trailer brake master cylinder is warranted if it is found to be half full of brake fluid in any section. The master cylinder is a critical component in the braking system, and its proper fluid level is essential for effective operation. A master cylinder that is at least half full may indicate that there is insufficient fluid to allow for optimal braking pressure, which can lead to brake failure or reduced braking efficiency.

Maintaining the appropriate fluid level is vital for ensuring that the braking system is fully operational and can respond effectively in various driving conditions. Therefore, a condition where the fluid is only half full raises a concern regarding the functionality and reliability of the braking system, justifying the rejection.

While it may seem that a full or leaking brake fluid level might also suggest issues, they don’t inherently compromise the operation as severely as the critical low fluid condition represented by being half full. Additionally, using the wrong type of fluid is a concern for the performance and compatibility of the brake system but doesn't directly relate to the rejection criteria based on fluid level.