Air brake question

[wilko]

I just went through a set of front brakes way too fast, so I'm either getting too much air to the front or the back isn't performing. The fronts are adjusted loose and aren't dragging. I notice the front wheels are almost too hot to leave a hand on, while the back wheels are barely warm.
I'm assuming that whatever pressure shows on the application gauge should match what I'm getting to the back cans, but what should the pressure be to the front axle? Any diagnostic tips here?

 

[lantraxco]

Air pressure is normally the same all around, the can sizes and slack lever lengths vary....

What are we working on, does it have tandem axles on the rear, and if so does it have a split air system? Anti-lock brake system?

 

[wilko]

94 T800 4-axle. I believe there's a valve at the front (and on the drop) that cuts back pressure, at least at lower application pressures.

 

[Mike L]

94 T800 4-axle. I believe there's a valve at the front (and on the drop) that cuts back pressure, at least at lower application pressures.

I believe your right about that valve but I cant remember the name of it. go to the bendix or meritor website. there should be some info there on about application pressures. have you tried to back the front brakes off and road test the truck to see how the rears feel?

 

[ohiofleet]

it is a proportion valve don't know if that is spelled right

 

[ohiofleet]

are your back brakes in adjustment if so how are your s cam bushings are the drums wore if all is good I would look at the supply side vale be it an abs valve or just a supply valve

 

[theironoracle]

Those valves your referring to keep the front axle from breaking before the rear axles. The valves have low crack pressures if I remember 7-9 psi before they open but after that pressure they apply with the same air pressure as all other air cans. Now in regards to them wearing out to fast, I was once told by a friction guy that the brakes that wear at the fastest are the ones doing the work! Well, I kind of understand the theory but there may still be issues with the front brakes. Has this been changed application to a dump truck? If so 5th wheel tractors have a valve that called a bobtail valve which keeps the rear axle air pressure low so it doesn't skid the tires easily when there is no trailer, so when you put a dump box on them and run them around without a trailer you get poor rear axle brake performance but bad front brake wear. There are many other things too, did someone put type30 cans on the f/a, what weight of lining was installed, are there heat cracks in the drums?......TIO

 

[nowing75]

I second theironoracle on the bobtail valve. The Company I work for now had a tractor converted into a flatbed but they never removed the bobtail valve. They hauled a 20,000lb trencher on it and complained it would not stop.

 

[wilko]

No bobtail valve on this truck. I got time to fiddle with it using an air gauge on a hose. The front and the back pressures match exactly to the application gauge on the dash all through the range. The drop axle gets about half of what the others get. Everything is adjusted up, well within spec and wearing evenly. I've been running this truck about six years, this front wear is new.

 

[turbo64]

The front brakes should last a long time, maybe 12 years. I never replaced front brakes in 32 years. 20 different trucks, all cummins but 1 3406 a model. That engine was a real problem child.