Help with semi brakes.

Sorry, this will be long. If you know me, you know I'm winded.

During harvest I had a deer jump out in front of me. I stomped on the brakes in the '97 Freightliner and didn't hit it. I noticed pretty darn quick that all of the brakes on the Dakota hopper bottom locked up but not the truck. I didn't think much of it because I was empty so it didn't brake like it would set with a full load.

Yesterday I went to pick up a new tractor with the same truck and the 35 foot flatbed with electric brakes. I've learned before to check the glad hands and make sure they aren't leaking because you have to release the trailer brakes even though there isn't a trailer. It messes up the truck braking for some reason. It has been this way since I bought it from a leasing company. Anyway, when I tried to load the tractor it pushed the semi. I had someone manually set the trailer brakes with the controller. The farther I drove home the more I noticed that the truck brakes weren't doing the stopping. I adjusted the boost of the controller but that didn't affect it any. If I hit the brakes hard the trailer brakes locked up. If I turned it down to low boost I could stand on the pedal and it slowly wound down to a stop. When I was about 30 miles from home I pulled off on a ramp to pee. I stopped, set the tractor brakes, and took my foot off the pedal. It wouldn't even hold it once it was stopped.

I noticed at the same time that with the tractor brakes set I could still hear the rear actuators moving when I pumped the brakes. From my way of thinking the brakes should be set all the way so you shouldn't hear the brakes moving at all with the pedal. Long story longer, here's my question: is there an air flow divider on that truck that could have gone out sending too much air to the rear and not enough to the tractor? All brakes are working equally and have good pad left. It doesn't have any leak down unless it sits for days. I'm puzzled. It reminds me of a hydraulic flow divider that has messed up.

I know this will sound like a dumb question but have you adjusted the brake slack adjusters???? It sounds like the brakes are just way out of adjustment.

If you have adjusted the brakes then there is one other explanation. The brake valves on the rear axles on a semi have a special feature built into them. What it does is makes it to where you can't apply air pressure/force and the parking brake spring pressure/force both to the brakes at the same time. So IF you have the parking brakes set, which actually dumps the air that releases the spring to apply the brakes. If you push on the brake pedal which applies the service brakes this valve releases the parking brakes and applies air to the service brake diaphragm. So if your hearing the brakes work while the parking brake is applied the valve is not working correctly. This valve is usually mounted right around the middle of the front drive axle. Then the front and rear brakes are teed together. With parking brakes on one section and service brakes on the other.

This function will do funny things when it quits working correctly. Like releasing the parking brakes but not applying the service brakes. That is what it sounds like is going on.

To check it take an air line off the service brake side of one of the rear brake chambers and see if you have air flow with the parking brakes set with the service brake/foot brake pushed. If you do not then the valve is not working.

Email me if you want to talk about it in person. I don't know if I explained it to where you can understand what is going on. So shot me an email if you have any questions.

JDSeller has makes some good points. Given the info you've provided I don't really know of anything else I could add to his comments.


That said, here's a link to a Bendix book on troubleshooting air brake systems. It'll give you a good idea what your looking at with the various types of valves, etc you'll encounter in a typical air brake system. Hope it helps.
Troubleshooting

sounds like you have a bad brake valve. The proportioning valve can be deleted too, I always hated that stupid thing myself. I've had bad dash valves in the past that would by-pass and release the spring brakes with the buttons pulled out. I'd start with checking for air flow at the exhaust port on service valve, then work my way toward the maxi pots.does it have abs or traction control?

I agree with the others on several points. First check the actual adjustment of the brakes. It should have automatic slacks on it but may not be working. It probably has type 30 chambers on the drive axles and the push rods should not move more than 2 inches with the brakes applied 1 1/4- 1 1/2" would be better. Does it have parking brakes on all four drive wheels? I assume it is a tandem. If it does not have parking brakes on all wheels you will hear brakes move even with the parking brakes set on the other axle. Parking brakes can also be weak if the big spring is broke internally it may still move the pushrod but not apply adequate force to hold the vehicle on a grade. There is not any type of flow divider in the system except for some double check valves but they will not cause the problems you describe unless you have a leak while stepping on the brakes. The truck has a bobtail valve installed if you need to push the trailer supply valve in to have normal truck braking when a non-air braked trailer is connected.

Have any of the maxi chambers been replaced by a complete unit instead of a piggy back assemby? Your push-rod travel may have been cut too short and your not getting full stroke,you will notice poor braking and not holding when parking brakes are set.

If your brakes are in adjustment, you have broken springs in the 30/30 piggy back,That will explain why you hear the brakes moving when you apply brakes with brakes set in the park position,and word of caution,, DO NOT EVER ATTEMPT TO TAKE THE PIGGY BACK APART, there are two sides to the piggy back, service side which applies the brakes with the foot pedal, and the spring side, that locks the brakes when you pull the dash valve,you can be seriously hurt or killed if you attempt to take the piggy back apart unless you know what you are doing, do as others have said,, check your brake adjustment first, brakes must be released to do this, build up air all the way, chock wheels front and back, release brakes, then check adjustment, if your brakes are adjusted correctly, them remove chocks , set brakes and try to move tractor, if it moves easily, you have broken springs in piggy back, is your tractor a single axle or tandem ?

At first it sounded like you were describing a bad BP-R1 Bobtail Proportioning Relay Valve.
This valve reduces air flow to the rear tractor brakes when there is no trailer to provide better control.

Then as I read on I realized you just have bad brakes
Here is my reasoning.

1) With a trailer hooked the trailer is doing all the stopping; you just did not realize it until you used a trailer with electric brakes.

2) When you were bobtailing the Bobtail Proportioning Relay Valve was reducing the air flow to the truck brakes. Your truck brakes are so weak you found that releasing the trailer brakes; that took out the Bobtail Proportioning Relay Valve; so it provided full air pressure to the truck brakes. This extra air pressure gave you better stopping power.

3) your parking brakes do not work. You can take all the air out of the system and the parking brakes should still hold. They are held by spring pressure. If the springs are not holding the brakes tight you better fix that first.

Now is the problem you do not have them adjusted correctly or your shoes/drums are wore down so much they will not hold I can not answer.
I do not think it could be a broken spring in the brake chamber or a wrong replacement brake chamber used as that would only effect 1 wheel. It sounds like you are haves equal problems with all wheels.
If the brakes are adjusted correctly and you have the right amount of throw on the chamber rod I would be looking for glazed shoes or wore down drums.

Keep it simple.
Start with the basics.
Without a trailer connected drain all the air out of the system and see if the spring pots are holding the truck firm.
If they are you have an air problem.
If they are not holding firm you have a mechanical problem.
From the sounds of the problem you are dealing with, flat ground and wheel chocks are mandatory before you try anything.

I vote for the slacks needing adjusting. Turn the nut to make the rod push into the chamber then back it off about a half turn. If it has auto slacks and will not back off( check at beginning of adjustment) then adjust up some and have somebody push on pedal to see what the stroke is. It should be no more than an 1 3/4 inch to be in spec. Better if it is about an 1 1/4. If it has the long stroke slacks ,yes there are some of those out there on the newer trucks. I'm not sure about those for spec. I do know it is like about 2 1/2 inches stroke.
Also see if the slack arm is out the same amount when they are set in the parking mode. If they are different you will probably find the spring is broke. If you take them off and they rattle when you shake them it is broke. You can go 2 ways replace the whole can or just the piggy back. One is about 50.00 the other is about 35.00. These prices could be off some since I haven't bought any in a few years. I bought several ahead so I would have them on hand.

Do not take the spring brake pods apart. Buy new ones. Steve

We had a 95 FLD120 Freightliner acting similar, and we did all of the regular fixes. It turned out to be a square valve behind and to the right of the fifth wheel. Standing behind the truck it was mounted along the frame. It is a small valve, not the proportion valve. I forget what it was called. They told us at the Freightliner shop that this valve would not cause our trouble. We replaced it and it fixed our problems. Dad had a set of blocked off gladhands he attached to the lines when he was running bobtail.

You got some real good answers below.It does indeed sound like an out of adjustment problem.And you still could have bad maxis with broken springs.If the complete assemblies(not just the piggyback) have been replaced before there is a possibility that the push rods are too long.The angle between the push rod and the slack adjuster should not exceed 90* when the brakes are applied.Too great of angle reduces the leverage to apply the brakes,and you lose stopping power drastically.IF YOU ARE UNFAMILIAR WITH ADJUSTMENT PROCEDURES,FIND SOMEONE WHO IS.There are several types of slack adjusters and the procedure is different for each.Then after they are adjusted,make sure the parking brake holds also.Mark

"I've learned before to check the glad hands and make sure they aren't leaking because you have to release the trailer brakes even though there isn't a trailer. It messes up the truck braking for some reason."

This isnt right! You shouldnt have to release the trailer brakes to have brakes on the truck. Sounds like someone replaced/removed one or both of the valves and didnt put them back right to me.

don't mean to over simplify. .

I would suggest finding a good reputable truck shop and get them fixed. there is too much weight there to mess around if you don't (not saying you don't) know what you are doing. not worth the risk of getting hurt or hurting someone else (or worse).

a truck was dropping a trailer across the road from our office. Driver hopped out to get something out of the side box. DIdn't set the park-brake.

Truck started to roll down the hill. He chased it and jumped on the step. Truck went across the road, luckily no one was passing by, and into our yard and side-swiped a tree. The tree knocked him off and hurt his head, back and leg. Took him in an ambulance. It was 2-3 days later before they came and got the truck with a tow truck. I figured he would have been back later that day to get his truck.. I guess he was injured worse than we thought. Be careful.

this is to John in La, what part are you in, I bet I am not to far from you

Hello notjustair,

Our company trucks had what was called a bobtail valve. It made the brake less aggressive for the same amount of pressure that it applied the brakes while Bob tailing. Back to normal when hooked up to a trailer. If the valve is stuck in that position, the trailer brakes will come on first every time. May be just a bad foot valve, they do go bad. You need to have the air pressure reaching the brake chambers checked, on all the wheels!

Guido.

All good suggestions. If I may add one more: If you or anyone ever replaces a spring chamber, please do the unsuspecting a favor and use a torch to nip the spring in each coil to relieve the pressure it has. We always cut the springs at work before tossing them in the roll off for scrap steel. I have seen some at scrapyards just banging around on the ground, getting run over by a loader pushing up the tin pile. Some are so rusted that the spring is being held in by a thread, you wouldn't want to be walking or working around that if the spring lets go.

Ross

Bedico

I'm no trucker or tractor mechanic, but I do have a lot of experience inspecting air brakes as a DOT Inspector. If you by any chance have automatic slack adjusters and are depending on them to keep your stroke length within limits, be advised that they are not an "install and forget it" system. Every single ASA manual I looked at plainly states that you have to do the same regular slack adjustment inspections as with the non-auto systems. The ASA just takes up some of the variance between checks. Might be something to look at.

I would wager that your spring brakes are rusted, corroded, broken and causing the service brakes to not work properly.