'77 Mack S-Cam Rear Brakes

1977 Mack RS600L 38k rears Camel Back with S-cam rears.

It seems that the brakes in my rear axle in the tandem bogie are wearing way faster then the front tandem axle. I have the service history for the past 8 years and 80k miles. Both sides of the rear changed twice in that time. No brake work on either the front bogie axle or steering axle. Steering axle is a wedge system, but gets a lot of attention as the truck seems to break wheel studs on a regular basis, so the drums come off and inspected.

Any ideas?? Is this from the rear axle being the main drive axle?

It has the older style inboard drums, and was wondering if it's worth changing over to the outboard style.

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)

So what can cause the differential in braking power? Air proportioning valve? Bad and clogged air lines to the pots?

Do you have park (spring applied) brakes only on the rear axle, if so they could be not fully releasing.

norite said:

Do you have park (spring applied) brakes only on the rear axle, if so they could be not fully releasing.

Click to expand...

That is a very good question. I'll have a good look.

Spring brakes seem to be fully releasing. When on flat ground, the truck will feel 'free' and move a bit when park brakes released.( spring brakes pressurized )

Maybe a partially blocked air hose feeding the front drive brakes.

Could be the Quick release valves, had a problem years ago , there is a different opening pressure for some valves, and most braking only uses a low pressure, we weren't applying enough pressure to one axle , sorry i don't remember, maybe it was trailer valves were different to give a delay or something. I have seen where drivers only adjust easy to get to ones, different brake shoes, worn cams and bushings on one axle.

Art_H said:

Spring brakes seem to be fully releasing. When on flat ground, the truck will feel 'free' and move a bit when park brakes released.( spring brakes pressurized )

Maybe a partially blocked air hose feeding the front drive brakes.

Click to expand...

When you stop a vehicle traveling forward, weight tends to transfer towards the front. In a car or p/u truck typical braking ratio is approx. 60/40 which means the front axle does about 60% of the braking and the remainder is done by the rear.

If the rear drive axle is getting hotter it is dissipating more energy than the front drive. Perhaps the front slack adjusters are not set correctly, brake pads/drums are worn, even different tire diameters/air pressures between the two axles. Do you have larger than stock diameter tires on the steering axle? Is the truck loaded when you do your test? Is your rear drive axle locking up and the front drive not?

That's all I can think of at the moment.

overworked said:

Could be the Quick release valves, had a problem years ago , there is a different opening pressure for some valves, and most braking only uses a low pressure, we weren't applying enough pressure to one axle , sorry i don't remember, maybe it was trailer valves were different to give a delay or something. I have seen where drivers only adjust easy to get to ones, different brake shoes, worn cams and bushings on one axle.

Click to expand...

So just replace both then??

norite said:

When you stop a vehicle traveling forward, weight tends to transfer towards the front. In a car or p/u truck typical braking ratio is approx. 60/40 which means the front axle does about 60% of the braking and the remainder is done by the rear.

If the rear drive axle is getting hotter it is dissipating more energy than the front drive. Perhaps the front slack adjusters are not set correctly, brake pads/drums are worn, even different tire diameters/air pressures between the two axles. Do you have larger than stock diameter tires on the steering axle? Is the truck loaded when you do your test? Is your rear drive axle locking up and the front drive not?

That's all I can think of at the moment.

Click to expand...

Yeah truck was loaded when I took the temps.
Front is Wedge brakes...Yeah I know. lol But they may only start really applying with higher brake app pressures per design.

Front drive slacks are well adjusted, and drums/pads are also better than the rear drive axle. I just did my 6 month safety inspection, where the mechanic said that my rear shoes were getting down. That started me looking at the service history.

The temps I took I think really just support the observation that there was really only brake done to the rear drive, twice each side, in 8 years and 80k miles.

I suspect this is a larger problem than people realize. It's not a really squeaky hinge kind of problem, but I have a lot of hills to drive, and want all axles breaking hard.

Thanks guys for all the info and ideas.

I want to get a reliable method of supporting my dump box, for better access to the rear axles and air plumbing.

Were all of the shoes replaced with the same part number shoes at the same time? I have seen that happen when the shoes were replaced at different times from diferent suppliers. A little different material in the shoes and they wear and act totally different! Had one truck that the front axle shoes out lasted two sets of rear axle shoes. Finally convinced boss to put all shoes in at same time and they are back to wearing just about equal.

Are all the slack adjusters installed with the same center to center distance?