Does Master Cylinder of Actuator Need to be Bled when Replacing Actuator

Published 08/24/2017

Question:

Demco Hydraulic Brake Actuator - Drum - Zinc - 8inch Adjustable Channel Down - 8,000 lbs Item # DM8759131 Is the master cylinder bled already? Will instructions for bleeding break system and master cylinder be included with this part. I am replacing an old actuator. Thanks...

asked by: Mike F

Helpful Expert Reply:

You will still need to bleed your brake system when you install the part # DM8759131. It's not actually the master cylinder that needs to be bled as much as the brake lines. When you open the brake line to install the actuator you will be allowing air bubbles to get into the lines which is what bleeding the brake system gets rid of.

When you bleed the brakes on your trailer after installing the actuator, you will want to start with the brake that is furthest away from the actuator. Of the two bleeder valves, you will want to use the bleeder valve that is on the top. If you are manually bleeding the brakes, it is easiest to use a clear jar to collect the brake fluid in so it is easier to see when there are no more bubbles in the fluid.

Be sure to open the top bleeder valve when you are bleeding the breaks. If the brake calipers have a 2 piece bleeder, you will want to ensure that the upper fitting is loosened to bleed the system. I have attached a photo showing the two piece bleeder. Additionally, you will want to make sure you do not drain the reservoir of brake fluid at the actuator.

I have attached a video showing the process of how to bleed hydraulic brakes. I also attached a disc brake kit installation video that addresses bleeding the brakes at the 3:00 mark.

If you have taken these steps and are unable to bleed the breaks, I recommend checking the other brake assemblies to see if there is a potential block in one of the break lines. If none of the break assemblies will bleed, the break lines can be detached at each of the fittings on the break assemblies. With the lines detached from the fittings, attempt to bleed the breaks. This will help us determine if there is a blockage in any of the lines leading to the break assemblies.

If bleeding the breaks while detached from the fittings doesn't work, try removing the break line from the back of the master cylinder. With the break line removed from the master cylinder, try bleeding the breaks to make sure that the master cylinder bleeds properly.

expert reply by: Jameson C