How To Bench Bleed a Step Bore Master Cylinder
Step-by-Step Procedure to Bench Bleed a Step Bore Brake Master Cylinder
When installing a master cylinder, it is important to get all the air out of the system. If you don't, it can cause poor brake performance. A soft or non-responsive brake pedal and you might need to do the job again. Step bore master cylinders are more difficult to bench bleed than conventional master cylinders. They use an internal valve called a quick take-up valve that traps air causing difficulty in bleeding. Step bore master cylinders also have a larger primary bore, which takes longer to refill while bench bleeding.
The preferred method of bleeding a step bore master cylinder is to use a brake syringe such as the Cardone master cylinder bench bleeder tool. To bench bleed using this tool, the first thing you want to do is mount the master cylinder in a vise by clamping on to the mounting flange. Make sure the mount is level and avoid clamping under the casting body, since that could damage the bore. Next, place a drain pan under the master cylinder to catch any fluid that is expelled during the bleeding process. Then, remove the protective caps in the primary brake line port and fill the reservoir with fresh, clean brake fluid, at least halfway. Draw some fluid into the syringe and then purge the air from the syringe. Starting with the primary in-line in the reservoir, inject brake fluid until clear, non-aerated fluid flows from the primary brake line port. Then, refill the syringe, purge the air again and inject the fluid into the primary brake line port, till no air bubbles are visible in the reservoir. Do the same for the secondary in-line and secondary brake line port and complete the install following your OE approved service manual.
If you don't have a syringe available, you can still do a traditional style bench bleeding procedure, which requires stroking at the piston and is much more time consuming. There's a slight variation between bleeding a step bore master cylinder and a conventional master cylinder. So, pay close attention to the process.
Again, mount the master cylinder in a vise by the mounting flange keeping it left. Then, install the supply plugs into the primary and secondary brake line ports of the master cylinder and fill the reservoir halfway with fresh, clean brake fluid. Using a blunt tool, depress the piston using a short stroke of an inch or less and hold for 20 seconds. Make sure the stroke doesn't exceed one inch or else you could damage the seal. Keeping the piston depressed is the key to getting all the trapped air out of the quick take-up valve. It's important to wait at least 30 seconds between strokes, to allow the larger primary bore to refill the fluid. Repeat as necessary, until no air bubbles are visible and the piston travel is less than 1/8th of the original stroke. Finally, complete the install following your OE approved service manual.
As you can see, the Cardone bench bleeding tool definitely helps speed up the bleeding process, saving your valuable time and eliminating the chance of having to do the job twice.