I have a 2003 KW T 300. Rated at 26GVW. I dont think the rear brakes are working right. What controls the amount of air pressure going to the rear brakes?. It has new drums and shoes on rear. Front are shoes are good. Seems to be braking more on the front than the rear. When trying to make a panic stop (empty) the front will lock up but the rear will not. I have talked to other guys who run 26 gvw trucks with air and they say their rears will lock up. Is there a control valve somewhere? Does it need adjusting or replaced? I rhink it has anti-skid brakes on it but no indicatilon or problem. Help appreciated.
Your truck should have ABS so the brakes should not lock up. A simple way to check is to disconnect the service line from a rear brake chamber and plumb in a gauge to see how much air pressure you get with a full application.
Are your slack adjusters working ? Bob
Are the rear foundation brakes worn out? My fleet of T300/370’s go through rear axle brakes like crazy. They don’t like to hold on hills, either. Converted them to long stroke 30/36’s.
start by checking the stroke of the rear brakes have somebody push the pedal and see that the pushrod moves about 1 to 1.5 inches
Yes slack adjusters are set right. What should the air preassure be at brake chamber? Should it be the same as what gauge in cab says. Other trucker say theirs will lock up. Fully loaded at 26 k you had better give your self plenty of room to stop. I will try and plumb a test plug to check air preassure at rear chamber. One thing I did notice when going about 5 mph on gravel and hit the emergency valve it will lock rears up and stop right not but won't do that just pressing on foot pedal.
That does not sound right, sounds like you need a pressure gauge to tell what is going on. The springs apply roughly 60 psi worth of brake force to the brakes.
The pressure At the chambers will be slightly lower than the gauge reading. With a gauge installed chock the wheels release the park brake and build 120 PSI make a full application and see what the pressure is. When you apply the brakes the reservoir pressure will drop so you should see 105 to maybe 115 PSI at the chambers.
A good inspection of the thermoplastic air brake hoses might be in order.
If there was a application gauge already in the dash you would see 20 to 30 psi applied coming to a stop under normal conditions. 40 to 50 psi under hard braking. Parking brakes use full reservoir pressure to release brakes. The cracking pressure on the relay valve from the treadle valve should be around 4 psi before any service brake action. Under light braking conditions for control usually 10 to 14 psi.
On the cam arm, the clevis is mounted closest to cam adjuster. It has another hole also. If you mounted the clevis in that hole, wouldn't it have alot more brakeing force?
The slack adjusters come in 1/2" increments on the length from center of S-cam to clevis pin location. Those are designed to match mounting bracket for air chambers. The rod needs to exit the chamber in a straight line as possible to clevis pin hole in slack adjuster. If you call a KW dealer they can tell you the slacks it came with from factory by vin #. And each model of slack part # has to do with S-cam dia, # of splines and length.
The bracket that holds the air chamber has two holes. It could be set down to next hole and slack adjuster has a second hole also. Would this give me a little more brakeing leverage?
A little.
Disclaimer: Giving advice about brake system on the web makes me cringe. Stopping (and steering) is more important than going in my book! I would be very careful changing the geometry from the factory setup. If your front wheels are locking up something is wrong with your ABS system and the rear wheels should only lock up if you apply the parking brake. The first thing I would do is build full air, shock the wheels, release the parking brake then take a paint marker and mark the brake chamber rod were it comes out of the chamber. Then set the parking brake and check, the rod should be coming out of the can an inch to inch and a halve and the angle of rod to slack adjuster should be 90 degrees (or very close), look for flex in the can mounting bracket and side load or movement of s cam shaft (looking for sloppy bushing or wear of shaft). If every thing looks right then I would take a long screw driver or pry bar and jam the brake pedal down (parking brake released and wheels still chocked of course) wedging it against the seat frame or console. If brake application still looks good then I would unplug a wheel speed sensor and take it for a drive to make sure you get an ABS light on the dash. That will tell you that the ABS module is alive and at least looking at the speed sensors. You may be getting good air pressure to the brake chambers at a stop but the ABS modulators could be doing something strange with wheel speed sensor input.
That's not true even with abs under hard braking rear wheels can intermittingly lock up. Because of lag time of air modulators can't dump air fast enough. He should be seeing a abs warning light. Semi tractor/trailer have two indicators not just one like he has. Through the years I don't know how many systems I found where the tractor has less cracking psi than trailer or the opposite. People wonder why one set or the other wears brakes faster. There is also a factor in slack adjusters, are they drum or stroke senescing.
John, So what did you find out?