weak air brakes - TheDieselGarage.com
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post #1 of 20 (permalink) Old 01-26-2008, 01:14 PM Thread Starter
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weak air brakes

1955 chevy cab on 1996 freightliner FL70 chassi, my air brakes dont "feel" right, im new at the whole air brake thing...! i have very little mid range brakes, the brakes work for light braking or emergency hard stops and almost lock up. mostly rear brakes, very little front. did i missconnect the hoses, also the computer for ABS is no longer used but was told the ABS wheel controllers are in the normally open position so it doesnt affect anything having them still inline. any diagrams out there for the freightliner air brake foot pedal controller...the truck was a tractor orriginally, not a box truck.
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post #2 of 20 (permalink) Old 01-26-2008, 06:39 PM
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Automatic slack adjusters? Get under there and see where the adjustment is at. Make sure the fronts work at all.
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post #3 of 20 (permalink) Old 01-26-2008, 09:05 PM
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I'd check the slacks as well. I can always tell when mine need adjusting. The truck won't stop as good. I run them up tight, then back off 1/3 of one turn. A 1/4 is a bit tight and 1/2 turn is sometimes to much. Spring brakes will sometimes let a tractor roll then, 1/2 turn back.

If the shoe linings are glazed from excessive heat, cracking and separating from the plate is something else to look at.

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post #4 of 20 (permalink) Old 01-27-2008, 02:08 PM Thread Starter
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hi, i was hoping to find a web site or someone with knowlegde of the brake control hose hookup...since i switched cabs and did major changes i simply wanted to start from the beginning in my diagnostic, making sure my pressure hoses are connected to the right outlet on my foot pedal controller...i removed some hoses as they we going to the trailor hookup, the cab manual trailor override, there was some sort of " check" valve on one of the controller's outlet that is no longer there...
it seems my best bet is to go to a used truck dealer and look at the hookup on a " box" truck as it should be plumbed the same way mine should be right now...since i dont pull a trailer with air brakes...
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post #5 of 20 (permalink) Old 01-27-2008, 09:54 PM
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http://www.meritorwabco.com/brakecon...ortractors.asp

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post #6 of 20 (permalink) Old 01-27-2008, 10:39 PM
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Might want to look and see if that thing has a bobtail proportioning valve, and bypass it, or plug off the trailer supply and operate the supply valve if you still have that in cab to see if it changes the way it feels. Google the system or check out the bendix site for I think it is BP1 R5 valve. There is a simple way to bypass the operation of the proportioning part with a tee and some fittings.

That is a really cool project you have there. More pics & specs would be good!
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post #7 of 20 (permalink) Old 01-28-2008, 06:53 AM
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I would say somebody would have to look at the installation in person to see if it's done right. Where are you?

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post #8 of 20 (permalink) Old 01-28-2008, 07:55 PM Thread Starter
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ok, thanks to everyone for all the input, went and looked at a FL70 flat bed and my hoses are connected properly at the brake controller, doesnt mean i didnt mess up somewhere else but with the great web sites provided by BMOELLER i can figure things out, i removed quite a bit of " plumbing" since it was a tractor and some lines were ghost as they disapeared under the dash and into the walls of the cab and simply were cut or disconnected when i did the switch over, didnt want to tear stuff up to investigate where they went as i sold the whole FL70 cab...maybe i have a line crossover somewhere, even though both of my gauges do read different pressures and one tank seesms to stay full when the other ones does discharge when parked... but i think i have plenty of info to make progress and learn in the process...you guys are great, two heads..or more are always better that one...!
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post #9 of 20 (permalink) Old 01-28-2008, 11:28 PM Thread Starter
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just for fun and diagnostic purpose i set up a gauge on my front brake diaphragm, as i step on the pedal the gauge read about 5psi less than my tank pressure at full pedal, and the gauge climbs proportionally from 0 to full pressure relative to foot pressure...my max tank pressure is about 130psi
so all seems right according to that...im new at this, i expected air brakes to perform better and be more sensitive and stronger... hopefully its an adjustment issue...
i'll play with that tommorow...
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post #10 of 20 (permalink) Old 01-29-2008, 02:08 AM
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Yes, sometimes they just take some getting used to. The treadle acts directly on the rears and the fronts are relayed inside the foot valve so by putting the gauge on the fronts you verified that everything works properly as they are the last to get air.

The treadle valve is not a Freightliner part, most likely it is a Bendix part but it might be a little bit customized by FL but not much more than springs probably.

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