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killackeyptrcorp

Hydraulic brakes

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Greetings, I have a 2004 34’ Alpine coach. Recently we made a trip to the mountains, on return we made it safely down the mountain with no braking issues. Once we landed on a flat level roadway we came to a stop at an intersection, at the next stop the coach had ZERO brakes! Fortunately the coach was traveling only 10 miles an hour. Upon investigation we found brake fluid coming out of the master cylinder under pressure. I walked two miles to an auto parts store and returned with brake fluid which I promptly put into the master cylinder reservoir, we bled the brakes and were motoring down the road and made it safely back home about 100 miles away. 

The engine is 350 HP cummins with a properly functioning exhaust brake. During exhaust brake function the Allison transmission preselects a lower gear properly. 

Any ideas as to the cause?

Dave 

 

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killackeyptrcorp,

Welcome to the FMCA Forum.

When was the last time the brake fluid was replaced/totally flushed.

New DOTIII brake fluid boils around 500 degrees F.  As it absorbs moisture to protect the ferrous metal components of the brake system, the boiling point is reduced to 258 degrees F.

If/when the boiling point is exceeded (in the brake calipers) that fluid turns to a GAS.  That will push fluid out the master cylinder and at the same time severely reduce braking.

Strongly recommend doing a complete brake fluid flush.

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Wolfe 10.

Thank you for the welcome and too for the prompt response. Your diagnosis sounds very well thought out and reasonable. I was not aware of the heat conversation into a gas with brake fluid. That makes great sense considering the pressure behind the fluid coming out of the master cylinder. 

I own a diesel repair shop in AZUSA California, we specialize in diesel Engine and Allison transmission repair. Obviously we rarely do brake work. 

My guys just put it on the lifts to do exactly what you recommend. Thank you again sir!

Dave 

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Welcome!

Next time your going down a long hill, manually downshift, if your passing OTR's your going to fast.  You should know that.

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 Reach over on your right shoulder and shake your guardian angels hand for keeping you safe.

Many automobile companies recommend completely flushing brake fluid, Ford, and Mercedes Benz , and BMW recommend 2 yrs, other mfgrs vary..

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When you flush the brake fluid I recommend upgrading to DOT 4 or even better DOT 5.1 but not DOT 5 which is silicone based.  These have a higher boiling point than DOT 3 and recommended by many brake manufacturers.  DOT 5.1 is used in high-performance and heavy-duty applications due to its high boiling point. It’s compatible with DOT 3 and DOT 4 fluid   

There's also certain brake pads recommended for trucks such as semi metallic or carbon metallic but don't used ceramic brake pads on trucks.  Follow the link for more information on brake fluid.  

https://blog.amsoil.com/dot-3-and-dot-4-brake-fluid-whats-the-difference/

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