Initial shake down trip of a new-to-me XM818

98G

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Since I am driving an 809 series regularly on steep hills, I am curious what you found was the cause for your loss of brakes. I am in the process of replacing everything in the brake circuit, except the air pack. It has been functioning perfectly normally but maybe I need to rethink that decision. I do not know what the inside of one looks like, and didn’t think about seals failing in the air pack and making a place for fluid to escape.


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Cause not found. Upon arrival to AZ the master cylinder remains full and the pedal remains high and firm. I'd like to think it was just a bubble that was purged....

The thing about the air pack is that it's about the only place for the fluid to go where you wont see visible evidence of a leak. That's what makes it my primary suspect....
 

Robo McDuff

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Despite having gone through it thoroughly in June and paying special attention to the brakes, I lost the brakes entirely, entirely without warning. Pedal to the floor, no response.

No crash. No damage. No injuries. Downshifting got me slow enough that my newly installed park brake stopped it.

Bleeding the brakes restored pedal and apparently restored full functionality. I didn't get any noticeable amount of air out of any of the bleeders.

Obviously this is a bandaid fix on a symptom of a more subtle problem. I'm stopped in Socorro NM until it has been resolved to my satisfaction. I dont yet understand what happened and "anything that you dont understand is dangerous until you do".

My suspicion is the air pack. I'll be disassembling it for inspection and rebuilding/replacing as appropriate.
If the brakes on the old type tractor fails, should not the new presumable air-braked trailer still have at least some braking power or lock the brakes immediately as well? This is my biggest fear: brake system failing with a single-circuit system.
 

98G

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If the brakes on the old type tractor fails, should not the new presumable air-braked trailer still have at least some braking power or lock the brakes immediately as well? This is my biggest fear: brake system failing with a single-circuit system.
The semi trailer is also air over hydraulic.

But yeah, it should at least provide a 2nd circuit.
 

Lovetofix

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If the brakes on the old type tractor fails, should not the new presumable air-braked trailer still have at least some braking power or lock the brakes immediately as well? This is my biggest fear: brake system failing with a single-circuit system.
That is what I have thought too about pulling a trailer, I have a M1061 (5ton flatbed). But if 98G’s pedal hit the floor, then his trailer brakes (I presume they were functional and hooked up) should have locked up.
So is it pedal position that controls the amount of air going to the air pack piston, and also to the service connection for trailers, or is it brake fluid pressure from the master cylinder? I think it is the latter.

My backup plan for the event of a primary brake failure just evaporated. Now I need to find me a trolley valve and get it installed. ASAP!



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Robo McDuff

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Just did some back checking.

Truck is "air-assisted hydraulic"
Trailer is "air over hydraulic".

Two different things. 10+ years ago one member here gave a good explanation

This video is good as well

https://youtu.be/q4MtLobgbyQ

Remember: connection between truck and trailer is ALWAYS air (forgetting electric for now)

Correct me if I am wrong, but I think (not sure):

With full air brakes on the trailer, if the truck looses air, trailer automatically blocks the brakes AND the truck still has some brakes thanks to the hydraulic system.

With air over hydraulics on the trailer, it the truck looses air, the trailer maybe brakes but probably does not do anything.

If the hydraulics are completely gone on the truck, there is air pressure but the air does not get activated, so the trailer does NOT get a signal and does NOT brake UNLESS you have a functioning trailer brake mounted.

If correct, this last one is what happened here.
 

98G

Former SSG
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Just did some back checking.

Truck is "air-assisted hydraulic"
Trailer is "air over hydraulic".

Two different things. 10+ years ago one member here gave a good explanation

This video is good as well

https://youtu.be/q4MtLobgbyQ

Remember: connection between truck and trailer is ALWAYS air (forgetting electric for now)

Correct me if I am wrong, but I think (not sure):

With full air brakes on the trailer, if the truck looses air, trailer automatically blocks the brakes AND the truck still has some brakes thanks to the hydraulic system.

With air over hydraulics on the trailer, it the truck looses air, the trailer maybe brakes but probably does not do anything.

If the hydraulics are completely gone on the truck, there is air pressure but the air does not get activated, so the trailer does NOT get a signal and does NOT brake UNLESS you have a functioning trailer brake mounted.

If correct, this last one is what happened here.
This correlates with my experience.

Thanks for the elaboration.
 
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