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NE KS | Ive got a 91' peterbilt that is giving me brake problems. When bobtailing the truck brakes are all or nothing. When you push the pedal they work just a little and as you keep pushing they will all of a sudden lock up, hence all or nothing. Makes it a real challenge driving the truck with out a trailer. With a load everything is fine because enough weight is on the truck the tires won't lock up.
I've replaced the valve under the cab for the brake pedal (that seem to help the problem a little) , 2 of the brake chambers are brand new, and the other two aren't that old. Any one have any advice? I hate using up my trailer brakes to stop. All of the brakes are working, but to get them to lock up you've got to put the pedal almost on the floor. |
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Central ND | That's the way they are going to work when bobtailing, every truck we have is like that. Most of them actually have a decal on the dash or somewhere that says something like 'increased brake pedal effort may be requires when bobtailing' or something to that effect. |
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NE KS | jd4930 - 1/28/2018 20:55
That's the way they are going to work when bobtailing, every truck we have is like that. Most of them actually have a decal on the dash or somewhere that says something like 'increased brake pedal effort may be requires when bobtailing' or something to that effect.
It started acting like this about 8 months ago, I understand what your saying but this is really bad. I mean you have to go 3/4 pedal to get braking action, then a t .76 pedal travel the wheels lock up. When pulling a trailer at normal braking you can feel the trailer doing all the braking.
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Central ND | Ok, gotcha, I didn't realize it is worse than normal. Hopefully someone with a little more knowledge will chime in. If you can't feel the truck doing any braking action then there is something else going on.
Edited by jd4930 1/28/2018 21:03
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North Carolina | When mine done that way back when I had multiple trucks. It was always the regulator valve ( think that is what it is called) over the rear axles. This valve will have 4 lines going to the service side of the chambers and will be plumbed into a supply tank it will also have a line from the foot valve It is not letting enough sir from the tank got to the chambers |
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Mount Vernon, WA | Either your truck needs one of these: www.bendixvrc.com/itemdisplay.asp?documentid=5505
or it has one and it's not functioning properly.
Edited by Bern 1/28/2018 21:12
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West central Minnesota | What is the brake stroke at the slack ? should be under 2". if that is fine I would guess the bobtail proportioning valve is bad. mounted on crossmember right in front of fifth wheel plate. does it have auto slacks or manual ? Bendix bp-r1
Edited by flying61772 1/28/2018 21:14
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Marshfield, WI | Some of the older tractors before antilock brakes have what I think is called a proportioning valve on the brakes for the drive axles. When bobtailling they cut the amount of air going to the rear brakes on the tractor with the idea of helping to keep them from locking up on a guy. Might be something to look at. On our ‘96 Pete that valve sits right behind the fifth wheel plate on one of the cross members. |
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| +1. That valve can be sticking., and hence not modulating like it should. Also, verify the brake camshafts are greased and turning freely. |
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| Bern - 1/28/2018 22:10
Either your truck needs one of these: www.bendixvrc.com/itemdisplay.asp?documentid=5505
or it has one and it's not functioning properly.
file:///C:/Users/User/Downloads/031067.pdf
might want to check for pressure feeding the control port
needs to be full system pressure, with the yellow and red buttons in |
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SouthCentral WI | The simplest way to check it, is to tee an air pressure gauge into the service line of the chamber. When stepping on the brake pedal, presure should crack at 4 to 8 psi and increase slowly as you push on the pedal more. If this happens (correctly) then your issue is in the mechanical operation of the brakes. If the valve cracks and goes to 20 psi or higher then tee into the control line of the relay valve above the axle. This should have a slow increase of pressure as you push harder on the brake (treadle) valve.
I'd doubt you have a BR-R1 valve. They aren't that common. You can tell because it will have an extra line to it. It takes the place of the r12 valve, but requires a bit more plumbing.
My guess is a bad relay service valve (probably an r12)
Towmman
Edited by towman2000 1/28/2018 21:31
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 Bendix Air System.jpg (61KB - 326 downloads)
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| Try plugging the glad hands, then push the trailer valve in and run it. this works for my 97 IH with a similar problem. |
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North Carolina | Yep that is the valve I was referring to towman has correct terminology for it. Those things will drive you nuts if you have never experienced the symptoms before |
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East Palestine, OH | Take two glad hands, thread 2 pcs of pipe 6" long plugged on one end into the glad hands. Hook to air lines on tractor. When bobtailing you can charge trailer. It's what we did with ours, helped. |
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NE KS | Thanks guys I’ll take a look at the valves tomorrow. Truck has almost new automatic adjusters and everything is greased up good and working as far as the brakes themselves. |
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Nusbaum Farms LLC Bellfountain, OR | I think you mean the relay valve. That is what I would replace next.
-Orin |
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| steerstopper - 1/28/2018 22:34
Try plugging the glad hands, then push the trailer valve in and run it. this works for my 97 IH with a similar problem.
That should work while bobtailing ,
but how will it help when he is having trouble while the trailer is hooked to the tractor ?? |
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Lancaster, Co. PA | I had a similar problem with my T300 single axle. Brakes would be very soft and then slam on. You could hear a slap when they came on hard. Fronts would grab so hard that it would throw you to one side or the other. Really unsafe!! I was at Kenworth dealer getting parts one day and road service guy was there. Asked him and he said that's how his T3 service truck is. They don't know why, that's just how it is. Said to let HIM know if I figured it out!!
A week later I had a slow day so I pulled front wheels and drums. Turns out that the S cams and the rollers on the shoes had enough surface rust on the that they would not roll and engage smooth. Took full system pressure to make them move instead of coming on gradual. Sanded all the rollers and S cams and lubes with silicone grease now brakes work great. Didn't bother to tell Kenworth yet. |
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| hillfarmer the op said " everything is fine when hooked to a trailer" |
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| I read it as he was not wanting to wear out the trailer brakes early ?
so that is was a issue when hooked up
my 95 fld is terrible bobtailing
my 2000 Volvo does not matter bobtailing
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Central ND | steerstopper - 1/29/2018 13:03
hillfarmer the op said " everything is fine when hooked to a trailer"
True, but further down he added that they don't work right even with a trailer, it's just that they don't skid with extra weight, the tractor still isn't braking properly |
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NE KS | What I was saying was the problem was still there with the trailer hooked but wasn't as noticeable because of the trailer doing most of the braking. Also if I needed to stop quickly in an emergency the brakes are still working with full force and with a load they would most likely not lock up.
Today I replaced the valve behind the fifth wheel. I did not have an r12, mine was an OR65146 and while I was at it I also replaced OR28971 infront of the fifth wheel (I broke the mounting tab when shortening the frame so it needed to be replaced anyway). It will be a little while before I have the truck out to test it. the OR65 was $200 so defiantly not cheap.
That is interesting about rust on the S cams, I would never think of that. If the valve doesn't fix it I will look into the S cams. The truck very rarely runs in the salt and sits inside 95% of the time. |
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| So is it one of the valves that has th e1/4 inch "control" ;line
and did you happen to check to see if the 1/4 inch line if it has one had flow with the red and yellow buttons n
want to use wheel chocks while checking |
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West central Minnesota | 65146 is a BP-R1, so make sure the 1/4 control airline has pressure when red knob is pressed in |
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NE KS | I didn't check but I will. |
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Somerville, Indiana | ;should mention that its a good idea to modulate the brake pedal by applying with toes of foot and vibrating the action, fills all chambers the same if all valves are operating...Should be standard operating procedure, get in the habit and all goes well on ice and snow. |
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sw corner ia. | this might not be your problem, but it sounds like what happened to me..
take off your front brake drums and check that they aren't warped. mine were and really wanted to lock up when you just about come to a stop. new drums and this all went away. |
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NE KS | It is only the drivers that want to lock up, not the front. |
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| sound slike the 65146 was not working like it should after the trailer was hooked up
I hate the way the brakes work when the builder uses the 146
have a couple trucks without and they stop nice all the time |
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sw corner ia. | so you are sure the tandems' drums are ok? I am serious, this is what they act like.. it is easy to check. |
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Peterbilt brake issue
