Problem: Less effective bench bleeding causes air to enter hydraulic system.
Cause: Air left in the no-pressure areas of a master cylinder migrates to pressure areas once master cylinder is installed.
Solution: Apply unconventional vacuum bench bleeding technique to insure all air is removed during bench bleed process.
Step 1. Mount the master cylinder in a vise with the outlets at the highest point. Install vacuum bench bleeding kit or brake lines at outlets and depress the master cylinder piston about 1/8″. Hold the piston in the depressed position. Make sure to grip against the snap ring and not the cylinder bore. This step changes the fluid path.
Step 2. Use a vacuum bleeder to apply suction at the secondary outlet while holding the outlet hose in an upright position. If we apply vacuum with the piston in this position the vacuum will collapse the primary cup seal and suck fluid through the replenishing port into the low pressure chamber. Once it fills up the fluid will pass over the cup seal and fill the pressure chamber and then out the line fitting.
Step 3. Repeat vacuum bench bleeding process on the primary outlet port.