Brake Pedal Sinking Before a Workshop Visit
Your diesel pickup has done 95,000 km. At a red light, the brake pedal slowly sinks toward the floor when you hold steady pressure; it firms up if you pump it once. The brake fluid reservoir is at the MIN mark, and you find a dark wet patch on the inside of the right rear tyre. The handbrake lever pulls up almost to the end of its travel. The ABS warning lamp is not lit. You are 10 km from home and 2 km from a workshop, and you have a passenger. The brakes worked normally on the previous trip, and the pedal feels normal again after pumping. The workshop closes in 30 minutes. You are on a busy two-lane road with no hard shoulder.
1. Diagnose the most likely fault and explain why pumping the pedal helps.
Read the worked answer
The sinking pedal and wet patch point to a hydraulic leak, most likely at the right rear wheel cylinder or caliper, with air entering the system. When you hold pressure, fluid escapes, so the pedal falls; pumping temporarily restores pressure by pushing more fluid, which is why it firms up. The reservoir at MIN confirms fluid loss. The handbrake travel suggests rear brake adjustment or contamination by fluid. This is not a monitor-it fault: a sudden complete loss of brakes is possible. The ABS lamp being off does not rule out a hydraulic fault.
2. What should you do about the 2 km journey to the workshop?
Read the worked answer
Do not drive the 2 km. A brake system that cannot hold pressure is unsafe at any speed, and the failure may worsen suddenly. Call a tow truck or a mobile mechanic, or have the vehicle moved by a qualified recovery service. If there is an absolute emergency and no recovery is available, use a very low gear, hazard lights and a route with no traffic, but this is a last resort and not a recommendation. With a passenger aboard, the risk is not yours alone. The 30-minute workshop closing time is not a reason to drive an unsafe vehicle.
3. What checks and repairs should be completed before the truck returns to service?
Read the worked answer
Before return to service: pressure-test the hydraulic system, inspect all lines and hoses, rebuild or replace the leaking wheel cylinder or caliper, check the master cylinder, replace the brake fluid, and bleed the system until the pedal is firm and does not sink. Adjust or repair the handbrake, check for fluid contamination on the rear shoes or pads, and road-test at low speed in a safe area. Recheck the reservoir after the test and look for fresh leaks. Also inspect the ABS sensors if the lamp later illuminates. Keep the repair invoice.