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Troubleshooting Master Cylinder T4749501042 Not Releasing Fluid When Installed on My Trailer


Filled master cylinder with brake fluid, brake line not connected, Hooked up to my truck and moved back and forth. No brake fluid comes out of the discharge. What do I do now?


Helpful Expert Reply:

Sounds like there could be a few things happening. Your coupler could not be depressing the piston of the master cylinder, your coupler has an electric lockout, or the master cylinder is bad.

Let's start with checking the coupler to make sure its getting enough pressure to the master cylinder piston, I recommend taking your safety chains from the trailer around the coupler and hook them up closely to the hitch of your vehicle, this way you will be able to use about a 4 foot 2x4 board to apply direct pressure to the coupler and see if the master cylinder has a discharge. It is important to note that you will really want to bleed the system very well after this because you will have introduced air into the system which needs to be air free to work properly to provide braking power to your trailer.

If you have an electric lockout on your trailer coupler then when you backup you won't get any fluid to the brakes because the lockout is designed to allow you to freely back up without applying the brakes. If you try the method outlined above this will be bypassed, you could also disconnect the electric lockout and try backing up.

If this still results in no fluid then it is likely you have a bad master cylinder and would want to replace it.

helpful expert reply by:
Jackie C

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