Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A technician is performing a brake fluid flush on a vehicle. The manufacturer specifies using a pressure bleeder set to 15-20 PSI. After attaching the pressure bleeder and opening the bleeder screws, the technician notices that the fluid flow is inconsistent and sometimes stops completely, even though the pressure gauge on the bleeder shows a steady 18 PSI. What is the MOST likely reason for this inconsistent flow?
- AAir is trapped in the ABS hydraulic control unit (HCU).
- BThe master cylinder reservoir is low on fluid.
- CThe brake lines have internal restrictions.
- DThe bleeder screws are partially clogged.
Show answer & explanationAnswer & explanation
Correct answer: C. The brake lines have internal restrictions.
Inconsistent or stopping fluid flow with steady pressure from a pressure bleeder suggests a restriction in the brake lines, preventing the fluid from flowing freely to the bleeder screws.
Why the other options are wrong
- A. Air trapped in the ABS HCU would lead to a spongy pedal or ABS malfunction, but not typically cause intermittent flow from the bleeder screws under steady pressure.
- B. If the master cylinder reservoir were low on fluid, the pressure bleeder would eventually push air into the system, not cause intermittent flow with constant pressure.
- D. Partially clogged bleeder screws would cause reduced flow, but typically not intermittent stoppage with steady pressure from above.
Brake Line Restriction Symptoms
A restricted brake line or hose impedes the flow of hydraulic fluid, leading to symptoms such as inconsistent fluid flow during bleeding, pulling to one side during braking, or a wheel dragging after pedal release. This occurs despite normal system pressure.
- Inconsistent fluid flow.
- Steady pressure applied.
- Can be hard line or flexible hose.
- Causes uneven braking.
Memory trick: Fluid's trying to pass, but a hidden block's in its class.