weak air brakes [Archive] - TheDieselGarage.com

: weak air brakes


gon2sea
01-26-2008, 02:14 PM
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.

The Ayatollah
01-26-2008, 07:39 PM
Automatic slack adjusters? Get under there and see where the adjustment is at. Make sure the fronts work at all.

bmoeller
01-26-2008, 10:05 PM
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.

gon2sea
01-27-2008, 03:08 PM
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...

bmoeller
01-27-2008, 10:54 PM
http://www.meritorwabco.com/brakecontrolvalvesfortractors.asp

togoffrd
01-27-2008, 11:39 PM
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!

Birken Vogt
01-28-2008, 07:53 AM
I would say somebody would have to look at the installation in person to see if it's done right. Where are you?

Birken

gon2sea
01-28-2008, 08:55 PM
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...!

gon2sea
01-29-2008, 12:28 AM
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...

Birken Vogt
01-29-2008, 03:08 AM
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.

Birken

gon2sea
01-29-2008, 12:11 PM
it feels like the rear brakes are the "primary" brakes, as they seem more than willing to lock up if i push hard, of course this could simply be because i do not have a "load" to put weight on the rear of the truck, or are brakes on big trucks designed that way as the rear brakes on the tractor serve the same function as the front brakes on a car..when brakes are applied the weight is shifted forward and down therefore they need to be the stronger ones as the trailer weight is now transfered to the tractor rear brakes and the down forces gives them more "grip" on the pavement , in a car or light truck, the front brakes serve that function and therefore are more powerful.
trying to learn here...lol...

gon2sea
01-29-2008, 11:18 PM
i think i found my problem... i have automatic brake adjusters and i seem to have WAY too much travel, i can pry the arm away from the diaphragm and it seems like the diaphragm is reaching full extention or close before the shoes make contact, so, my question is...how do i adjust the travel...every picture i have seen shows a standard arm, and that doesnt help me, is there a manual adjustment screw?

The Ayatollah
01-30-2008, 02:38 AM
Does it have the little 5/16" square thing on it? Raise the front wheels off the ground. Do they turn free? Hitting the brake, how far does it travel before the fronts stop turning?
I get the fronts to spin free, tighten until they stop turning (not jamming the linings into the drum, just stopping the wheel), back it off 1/4 turn, see if it spins free. From there, I go to where it won't quite freewheel but it's not really dragging, a lot of that depends on personal preference and what you haul but it should be near 1/4 turn. I do the 1/4 turn thing because it should be very close to the final adjustment I want. Initially tightening it also tells me if it was 1/4, 1/3, 1/2 etc when it came in. Gives me some idea if the slack has been keeping a consistent adjustment or not. Once you think it's adjusted, drive it and see. If you think a slack is screwed up, after the drive jack it up and see if one side is a way off already. My RF just went and after driving a short distance, my RF wheel couldn't be turned by hand. Glad I caught that early.

bmoeller
01-31-2008, 02:41 PM
http://www.youtube.com/watch?v=PXG4gjzEbDU

http://www.youtube.com/watch?v=2VfmH1XNE0c&NR=1

Birken Vogt
02-01-2008, 03:19 AM
That video on the slack adjuster references a different type (Bendix I think) and you will break a Rockwell/Meritor type (square adjuster nut) if you try to back it off by brute force like that.

Birken

bmoeller
02-01-2008, 03:58 AM
Is there a good way to back off the Meritor adjusters? Cranking on the teeth can't be good. That was the only thing I didn't like about it. Didn't have time to find a better one. Had to leave.

Birken Vogt
02-01-2008, 01:33 PM
There is a little button you pull on most of them, but the oldest ones have no such button to pull and you have to unscrew and remove the little thing instead. I think I like it better than the other companies' because theirs you have to use a 1/2" drive handle (for extra length) and when you crank on them to back them off it feels like it is going to break the thing.

Birken

bmoeller
02-02-2008, 06:40 AM
We have several trailers that you can use a screwdriver to hold the button out. Washoping there might be a better way for the other style. I hate cranking on them.

Birken Vogt
02-02-2008, 01:56 PM
We have several trailers that you can use a screwdriver to hold the button out. Washoping there might be a better way for the other style. I hate cranking on them.

It's no fun to back off the non-Meritor type, is it. Their manuals also state not to use air tools to back them off, or you will break them, but I have been sorely tempted a time or two to do it anyway. But never did in the interest of keeping things un-broke.

Years ago I converted some older fire trucks to ASAs. Meritor would have been my first choice but they don't give all the options you need for the different combinations of arm length and chamber size. I used Meritor on a C-8000 with 30s in the back and I think 24s in the front. A big Pierce had 30s on both ends and some longer than usual arms and Haldex is the one that has all the strange options for that sort of thing even though I am not real fond of their anchor bracket system. Another old F-700 had 16s in the front and the Bendix adjusters were the only ones that could be used for that application. Fire trucks are weird and sometimes ancient. But Meritor would be what I would pick anywhere they have an application. Just make sure to use the card to set them up properly.

Birken

keydl
02-03-2008, 05:32 AM
Pull the pin and snapring and set the adjuster back a spline on the cam.

Some of them only back up 2 times from a manual adjust before they start slipping adjustment.

Put a plastic can cover on the push rod at the face of the chamber and check the travel when the brakes have a light service brake application, that way you can put a stick on the pedel aginst the steering wheel or seat and look to check the brake adjustment. On spring brakes the distance that the rod travels should be the same for all wheels and the change is the amount that they are out of adjustment. They sell a patent set for $20 but peanut can lids work well. I like them for a quick check and the DOT that looked thought they were good, easy to put a tape to and measure the exact rod travel.

keydl