Brake System Failure

You can not underestimate the power of a VIS-Check. If you've never had one of these done, I can not stress the importance of this test strongly enough. Please get one done, especially if you've purchased your truck from a source other than a VIS-Check certified dealer.

I had suspected that my brakes were weak. This was confirmed in 30 seconds when I drove two other trucks including an identical 770. This compelled me to have a VIS-Check done during the national HDT rally in Wichita, KS in October of 2006. The results of the VIS-Check are shown below.

VIS-Check Front Axle

VIS-Check Rear Axle

The graphs above show two different pictures. The first is the graph of the front axle. It shows good brake attack, rising to maximum pressure, and a clean release. The graph of the rear axle shows that under full pedal pressure, the rear brakes do not apply at all. The maximum brake pressure is less than the rolling resistance of the wheel. One thing is certain, while a flat-line is well documented, this is first time anyone has actually seen a flat line response during a VIS-Check. My truck could not leave the facility until it was repaired.

VIS-Check of Brakes

VIS-Check of Suspension

The two pictures show the VIS Check machine itself. The first is a picture of the brake test. An electric motor spins each wheel at a fixed speed. The the operator is instructed to press the pedal fully and then release it. The restistance of the wheel against the roller is measured and then the graph of wheel performance is generated. The suspension test consists of placing the wheel on a shock table that produces violent movements in all three axial planes. While this test is underway the operator looks for play and motion in all suspension, steering, and bearing assemblies.

Rear Service Brake Line

Plugged Pedal Valve

Pedal Valve Assembly

Here's what's wrong. The service air line to the rear brakes has been cut and plugged at the valve body. The simple fix is to replace the brake line and while we were at it, I had the bobtail proportioning valve replaced with a standard rear service valve. If you have ABS, you have no need for a proportioning valve. Unfortunately, the ABS on this truck is disabled and once it receives a fault. ABS is currently inoperable, to be fixed at a later date.

Doonan's made emergency repairs so that the truck was safe to drive back toward home. I didn't want to pay full shop rate for a complete rear service brake overhaul.

While in Delaware, swapping out the rear axle (Details Here), I did a rear brake overhaul. The drums were questionable, the brake pads on their last legs, and I had to pull all of the parts off to rebuild the rear end anyhow, so it was a perfect time to replace the rear brakes. I didn't take many pictures as my hands were just too filthy to work the camera.

New Brake Drum

ABS Wheel Sensor

Parking Brake Air Valve

A brake rebuild is not that difficult, but it does require knowledge. I purchased the knowledge in the form of a heavy truck mechanic for 5 hours at $30 per, he worked side by side with me to complete the repair and teach me how in the process. I will do the front brakes next spring myself. The ABS sensors and emergency repair were abnormal. A full brake job on a working truck with all new parts is about $500 per axle if you do it yourself.

The only parts that I serviced and reused were the slack adjusters themselves.

Cost of Parts:

Emergency Service at Doonan's: $537

Brake Drums: $167.98

Brake Shoes: $85.20

Spring Kits: $16.98

S-Cams: $88.97

30/30 Brake Chambers: $87.98

ABS Sensors: $80.42

Rental Mechanic: $150

Total for Repair: $1214.53

Still Remaining: New ABS Module, Installed ~$1075