Brake Problems

[Indian Steve]

I have a 94 Ram 2500 with the V10. It has electric trailer brakes. In the the last 2 years {10,000 miles}, I have had the master cylinder replaced 3 times. This ONLY happens when pulling the trailer. I will be rolling along with a 16 ft trailer loaded with 1,000 -3,000 pounds and go to hit the brakes and the pedal goes to the floor. I can pump the brakes and tighten up trailer brakes to get home. No leaks at wheel cylinders or calipers or anywhere else i could see. Master cylinder is full. I took it to my mechanic and he replaced the master cylinder and everything was fine for a couple of hundred miles and then same thing happened. Took it back to mechanic and he replaced master cylinder. Everything was fine again for a couple of thousand miles and same thing happened. Yesterday, I was coming home pulling the trailer less than a thousand pounds and damn pedal went to the floor again. This has always happened when the trailer was hooked up and trailer brakes are working fine. What the Heck is going on?

 

[Indian Steve]

It gets even weirder. I bought a new master cylinder and when i went to move the truck to a better spot to work on it, the brakes are working fine again.

 

[Indian Steve]

After replacing 3 master cylinders I finally found the problem. The proportional valve was sticking. My master mechanic claimed it was a bad mater cylinder 3 times and replaced master cylinders that there was nothing wrong with.

 

[GTyankee]

Maybe your mechanic needs to go to school for braking systems ?
He should have known that master cylinders don't go bad that quickly & he should look for the problem elsewhere