Stamey

As I have been learning more about air brakes I have read that you should not operate the service brake when the parking brake is on if you have S-cam brakes. Apparently it can damage the parking brake mechanism.
I don't understand why the design would be this way since you would be expected to be operating the service brake when you engage or release the parking brake, so you don't roll.
Can someone explain this to me?

Thanks,
Chris

DmaxEter

When the parking brake is applied, there is a giant heavy spring in the brake chamber that is pushing aginst the slack adjuster which is twisting the s-cam and it is spreading the shoes out to the drum. When that spring is pushing on the slack adjuster and you push the service brake with a decent amount of force it is just putting twice or three times the normal pressure on the slack adjuster and its internals. I have never seen any damage caused to any of the compnents due to someone doing this but thats the theory or at least what I was taught.

Nick02Ram

I've never seen it hurt anything, either. But you're not supposed to do it. That huge spring (I have a crooked middle finger b/c of it) on top of 70-90 psi exerts a lot on the brake system, but I've never seen any damage, I could possibly see premature cam bushing wear maybe.

1-5-3-6-2-4

theres a valve in modern trucks thats prevents over pressure. from service application while chamber is dynamited. but you still shouldn't do it/.

dieselcamp

Newer trucks (last 20 years or so) most have anticompounding valves so when the spring brake is applied and you use the service brake it wont feed air to the service of the brake chamber. Dose this with a signal line frome the pp valve.

Nick02Ram

Yeah, i dunno how I didn't think of that. I replace em quite frequently, also have to bypass the anti-compound part of the valve once ina while on the side of the road. The SR-4's I think it is, get bypassed all the time.

Stamey

Well I was wondering because my 1986 IH S1900 gets parked on a hill, and when I am releasing or engaging the parking brakes I always have my foot on the service brakes to prevent unwanted roll, especially since I imagine it is not the best idea to engage the parking brakes while the truck is in motion.

Thanks,
Chris

94ram

its been said that all that extra pressure is enough to break the shoes. i always let off right before applying the parking brake or hop on it right after releasing it.

shorthair

For the past 30+ years I've applied the service brakes before releasing the Maxi's but as I only apply 10-20#'s nothing has broke yet. Even when running Diamlers test beds we didn't have any problems but if you are parked on a hill so steep that you have to apply more than 2-# then I suggest you are parking in the wrong place.

Clayten

Ya that is a for sure