I have a 1950 2 ton dump truck with a Bendix Series 42 hydrovac booster that was recently rebuilt. I asked the vendor to rebuild the master cylinder but they said it was too far gone and replaced it. We installed the booster and master and got the brake system bled then used the truck for a handful of material runs.

The master cylinder has leaked varying amounts of fluid into the boot at the push rod since reinstall but the brakes worked fine until they locked up on my last run; it seemed like the check valve may have locked up. Relieving pressure on the line at the booster from the master allowed me to get home. We're re-bleeding the brake system following the Chevy Truck Shop Manual. We continue to get air bubbles during the bleed and fluid leaking out of the boot at the push rod.

Thoughts? Would the leaking fluid indicate a bad seal in the master cylinder?