Problem: Master cylinder running dry during extensive hydraulic service.
Cause: When performing brake line or upper system component replacement the brake fluid will drain out of the system due to the master cylinder vent ports.
Solution: When performing extensive hydraulic service use one of the following methods to prevent the master cylinder or system from running dry:
TIP: It is a very good idea to prevent any air from entering the ABS modulator whenever possible. Some modulators require special tools and procedures to remove trapped air. Some non-ABS systems can also be very difficult to bleed out if the systems run dry.
Wheel based service (caliper or wheel cylinder replacement):
TIP: It is a good practice to drain the reservoir and fill with new brake fluid BEFORE starting the repairs. Performing this step will allow the new fluid to replace the old fluid as any fluid drains during service. it is also wise if gravity bleeding is going to be used during the repair process.
1. Calipers: If the replacement caliper is available the calipers can be swapped with little or no fluid leakage. If not use a tire valve stem to plug the brake hose at the banjo bolt hole. A line lock could also be used to prevent the system from draining.
NOTE: Do not use line locks on older hoses that have lost their elasticity or internal hose damage could occur. NEVER use vise grips on brake hoses.
2. Wheel cylinders: On most models the wheel cylinders can be swapped without much fluid loss. If necessary place a line lock on the rear brake hose supplying that wheel before service.
Upstream Service (brake lines, valves etc.)
Replacing these components represents more of a challenge when trying to prevent the fluid from draining out of the system during service. Use one of the following methods:
1. Plug the outlets of the components involved in the repairs until ready to install the brake lines.
2. Use a brake pedal depressor or other suitable device to depress the brake pedal approximately 1/2" to 1" (see Figure 74.1). This process covers the master cylinder vent ports closing off the fluid path from the reservoir to the hydraulic system (see Figure 74.2 & 74.3).
Figure 74.1 
Figure 74.2
Figure 74.3NOTE: If the repairs are going to take a long period of time it is best to disconnect the stop light switch to prevent the battery from going dead.
