Maintenance Q&A
Pesky leaks and trailer brakes

By Paul Abelson, senior technical editor

Q. I have a 2001 Freightliner with an Eaton Fuller 13-speed transmission behind a Series 60 Detroit. I do minor service work myself but leave the big stuff for the techs at the dealer. I can’t afford all the special tools and fancy equipment they have. I was doing my preventive maintenance service back in December when I noticed some oil under the transmission. It wasn’t much, so I wiped it off and checked the oil level, which seemed to be OK.

At the next preventive maintenance, I noticed the same thing, only a little more. I checked again and topped off the oil, which took only a few ounces. In March, same thing, so I took it to the dealer. They said I need new transmission oil seals. The seals are reasonable, but there’s a lot of labor involved. I picked up the truck and everything seemed fine until my next preventive maintenance. The same leak was back. I went to the dealer and they said the work was under warranty. They said it was probably a bad seal and they replaced the front one again. In May, it happened again but the dealer won’t fix it again. Now he says there may be a crack in the housing. My options are to get a remanufactured transmission or a replacement housing and move all the insides over. What should I do?

A. Are you sure the dealer exhausted all possible causes? There is one you haven’t mentioned. When oil is churned, as gears or a crankshaft run through it it heats up and forms vapors. In pre-2010 trucks, these vapors were allowed to escape through a breather mechanism. Air could escape the heated engine or transmission, carrying oil vapor with it into the atmosphere. As the component cooled, air would be drawn back in. Often there was a mechanism – foam or baffles – to capture as much oil vapor as possible to return it to the engine or transmission. Over time, dirt and residue build up on the collection mechanism, sometimes completely blocking air flow.

When the breather is blocked, pressure builds up in the transmission. Your truck is 13 years old. That’s ample time for breather blockage to occur. The increased internal pressure could be enough to force oil past the seals. Check the breather mechanism for obstruction before spending any more on seals or housings.

If the cause is not the breather, we go back to your original question. Both repair and remanufacture have advantages and disadvantages. How long do you expect to keep your 2001 truck? You have a bullet-proof engine that is out of date for emissions and will need upgrading to run California and possibly other states. Your truck is well beyond warranty, so if the problem is the housing, getting a remanufactured transmission with a full warranty may not be that advantageous.

The answer will depend on your long-term plans for the truck. A simple repair with the dealer’s 90-day warranty may be sufficient to let you sell or trade the truck.

Q. Our fleet has been upgrading to new Kenworths over the past year or so. They all have front air disc brakes, and some of them also have air discs on the drive axles. I’ve heard you shouldn’t mix disc and drum brakes, but if KW does it on the tractor, I figure they tested it out. My concern is about the trailers. We have about three or four for every tractor and we do mostly drop-and-hook. The trailers are in good shape, but we keep them forever. None of them have disc brakes. Is it safe to run air disc brakes on the tractors and drum brakes on the trailers?

A. Your concern is instability from uneven braking. There are several causes. One is variance in side-to-side braking torque across any given axle. Another is front-to-back variation. We see side-to-side as a tendency to wander when the brakes pull more to one side or the other.

More violent movements were more prevalent before the turn of the 21st century. Depending on which sets of wheels locked-up first, we saw plowing when the steer axle locked up, jackknifing when drive wheels locked, and trailer swing when trailer wheels lost grip. When the tires with traction are slowing down, skidding tires tend to rush ahead.

To prevent this unbalanced braking, the National Highway Traffic Safety Administration required new vehicles to have anti-lock braking systems, or ABS, starting with tractors in March 1997 and trucks, buses and trailers a year later. In actual practice, many tractors and trailers were ABS-equipped before then. They had been proven and operators saw the safety benefits. By sensing lock-up and modulating brakes to prevent it, ABS all but eliminates the tendency for any set of wheels to lock up and lose traction.

The impetus to use air disc brakes came in 2011 with regulations that reduced stopping distances from 60 mph for class 8 trucks by 30 percent – from 355 feet to 250 feet. Truck makers had to increase brake size, effectiveness or both to meet the new standards. Air disc brakes are more effective than drums without the added weight of more massive drums. They have another significant advantage. They virtually eliminate side-to-side braking variations so there is less wander.

You state that your trailers are older. Are any from before 1998 when ABS became standard equipment? If so, you might want to check with your maintenance or safety managers about pairing older tractors and trailers, or about replacing any non-ABS trailers remaining in the fleet.

But with ABS, I wouldn’t worry at all about running disc-braked tractors with drum-braked trailers. LL


Senior Technical Editor Paul Abelson is a life member of OOIDA, holds an Illinois CDL, is active in the Technology & Maintenance Council, and is a recipient of the Lifetime Achievement Award from the Truck Writers of North America.

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