1977 Mack RS600L rear drive brake wear

[Art_H]

1977 Mack RS600L 38k bogie with s-cam on the drives, seems to wear brakes more on the rear drive, than the front drive. Just bought the truck a few months ago, and I have the service history from a Mack dealer. The guy really looked after the truck. But I did notice that there was never any history of front drive brake work in 8 years, and 80k miles.

Any ideas as to why??

I'm going to try and track down the previous gov inspection sheets to see what the linings etc were every 6 months.

I have the inboard drums as well.

[gearhead204]

check if truck uses 2 application relay valves, one could be bad or if it uses one relay check hose plumbing if front hoses use 90 fittings and rear don't that could be a issue also check slack adjusters for equal length also check size of chambers on both axles . have you watched the brakes apply? could s cams be installed bassawkards?

if every thing checks out plumb 150psi air gauges onto front and rear axles and note application psi on both axles if this checks out check brake lining maybe damn good lining on frt. axle and soft butter on rear.

[Art_H]

check if truck uses 2 application relay valves, one could be bad or if it uses one relay check hose plumbing if front hoses use 90 fittings and rear don't that could be a issue also check slack adjusters for equal length also check size of chambers on both axles . have you watched the brakes apply? could s cams be installed bassawkards?

if every thing checks out plumb 150psi air gauges onto front and rear axles and note application psi on both axles if this checks out check brake lining maybe damn good lining on frt. axle and soft butter on rear.

All good advice. I was thinking that this may be a common issue. But maybe not.

I had a quick look with an IR heat gun, after a good hwy stop. I got the readings from the drum lips exposed on the inside of the wheels, to get accurate temps.

Rear drives were about double the temp than the front drives.

Rear drive around 50 deg C (120 F)

Front drive around 25 deg C (77 F)

Thanks, will have a look to see if there are multi relay valves. The good lining on front may have been possible, but now with the temps, the rear is clearly doing the lions share of the work.

Slacks are all good just changed manuals all set.

Truck had been serviced exclusively by a Mack dealership, and they didn't pick up that in 8 years they have only serviced the rear drive brakes...twice. lol

[pmolitor]

My one does the ssme thing. Mack told me it was common with the older Macks. Does yours have parking brakes on both axles?

[Art_H]

My one does the ssme thing. Mack told me it was common with the older Macks. Does yours have parking brakes on both axles?

Yeah park brakes on both axles. Does your Rear drive wear more than the front drive?

I can see how Mack would maybe say that, but with new (replaced) air brake parts since 1977, I don't buy it. The system is too simple. There are factors like air hose routing and fittings that can get in the way of equal axle air pressures.

But a 50% difference in heat on the drums is a lilttel too much for me. It means my rear axle would lock up early trying to stop the whole truck.

Maybe if I tighten the slacks a notch on the front drive axle, it will equalize.

On another note, these IR heat guns are a great tool when troubleshooting.

Thanks for all the replies.

[pmolitor]

Yes, the rear axle wears out 2 to 1 compared to the front axle. I don't have parking brakes on the rear.

[gearhead204]

on a dump truck I can understand the rear brakes wearing out faster especially if dust shields are missing due to dirt entering the drums. But when thinking about the mechanics of braking on a camel back or beam arrangement the fronts should be doing the lions share of the work. As brakes are applied the front axle is forced down and the rear axle is lifted, the fronts should wear more and the back should be more prone to lock up and skid, thus more front brake wear and heat