M936A2 Brake Performance Help Needed

[Speedzilla]

Hey Guys,

Trying to help a friend diagnose some brake performance issues with his M936A2. I have no experience with wrecker variants of the M939 series, so I don't know exactly what they should feel like, but they seem weak. The truck has non-functional ABS, and it is rare that the tires will bark/skid at all under full braking.

I was first wondering if anyone with an M936 could chime in on the braking performance compared to either a M939 Cargo, or an M809 wrecker. I want to make sure I am not expecting too much out of the truck.

Second I was wondering if there were any common points of failure that would result in low pressure in the braking system. Tank pressure is good on the truck, and the brakes have been properly adjusted. We haven't put a gauge on the cans to see what the input pressure is yet, need to gather some different fittings for that.

Thanks!

 

[doghead]

My M819 will lock all the wheels at any speed, quite easily.

edited for correctness

 

[Monkeyboyarmy]

My 936A2 is the same way. I have to apply significantly more pressure to get it to stop compared to any other air brake truck. The M923's that I have driven apply pretty heavy brake pressure with very little peddle pressure. I have not tried to diagnose the wrecker yet but my thoughts would be the treddle valve. I'll have to disagree with you this time Doghead. The more peddle pressure, the more air pressure to the brake chamber for service brakes.

 

[Suprman]

The truck is heavy. It's around 36000lbs compared to a cargo truck at 21000. Not gonna stop on a dime. Air brakes have a half second or so delay when applied. Not as much on newer trucks but the M939 trucks have it.

 

[Swamp Donkey]

Air pressure does not increase with brake pedal pressure, with air brakes.

This holds true in most civilian systems, but not on the M939 trucks.

At each rear axle wheel there is a chamber for each pad. So you have 2 chambers per wheel: 1 positive pressure chamber and one double chamber (positive pressure /spring). When the brakes are applied with the foot pedal, each positive pressure chamber operates it's corresponding pad, so 8 chambers operate all 8 rear pads, using only positive pressure chambers. When the spring brake is applied, each spring chamber operates it's corresponding pad, but only 4 pads are used since there are only 4 double chambers.

In the event of primary system failure, the inversion valve takes over to allow some braking function. It basically changes the system over to a standard system like you're talking about by using the spring brakes to stop. Braking ability will be maintained, but weakened, since you'll only be using the 4 pads instead of all 8.

 

[doghead]

I deleted that portion of my post, after MBA mentioned it.

I was thinking of the parking brake, not service brakes.

Sorry for the confusion.

 

[Speedzilla]

In the event of primary system failure, the inversion valve takes over to allow some braking function. It basically changes the system over to a standard system like you're talking about by using the spring brakes to stop. Braking ability will be maintained, but weakened, since you'll only be using the 4 pads instead of all 8.

Is it possible that on this M936 that there is some failure resulting in the inversion valve being operational and only four of the eight rear chambers are functional? I can't remember if the inversion valve blocks out the front brakes or not, from memory it does. I know the front brakes are working, at least as much as the rears anyways.

 

[Suprman]

If you only had service brakes the truck would roll when parked. Drain the air and see if it rolls. Safely though. Or it might roll away.

 

[Swamp Donkey]

Is it possible that on this M936 that there is some failure resulting in the inversion valve being operational and only four of the eight rear chambers are functional? I can't remember if the inversion valve blocks out the front brakes or not, from memory it does. I know the front brakes are working, at least as much as the rears anyways.

The inversion valve would use the fronts and rear springs during a failure. Disconnect the input line from one of the single chambers on a rear axle. If air is present when the pedal is pushed, I doubt you have a primary system problem since the inversion valve would be transferring air to the spring brakes. You should be seeing a problem on the gauges if the inversion valve has to swap the system over.

I'm not big on preaching about the TMs, but they do have some decent brake troubleshooting work packages that are pretty thorough. I also really like the P2P program for air system diagrams because it shows flow patterns.

My first order of business would be to see if all 8 rear positive pressure chambers were receiving pressure, and the correct pressure, when the pedal is pushed. Pretty simple test with a gauge and the correct fitting. If this test passes, it'll rule out several things.

 

[86m1028]

Haven't driven my new to me m936a2 much but I haven't noticed a big difference compared to my m923a2 cargo.
As noted earlier there is a big weight difference, mine weighs just shy of 40k with chains & tools.

 

[74M35A2]

I know not what you asked for, but my M925A2 weighs 24,500lbs, and stepping hard on the brakes will put everybody into the windshield.


I have not learned the brake part yet because mine work well, but maybe you should check to see if there are any adjustments to be made, and also pull the drums to make sure none of the shoes are oil or gear lube soaked.

Wreckers are heavy, as indicated. I have had 12,000lb in my bed, and it still stops strongly, but the brakes will smell slightly like they are pretty warm or hot after doing so. If they do not get hot like this after hard usage, I would suspect contaminated pads.

 

[Swamp Donkey]

Oops. Had too many windows open.

 

[red]

Could be an airline partially blocked (ice, dirt,etc). The old hydraulic brakes on my m816 wrecker would lock up all 6 395's if I stomped on it and the m939 airbrakes are supposed to be even stronger.

 

[Csm Davis]

My 936A2 will bust you head on the windshield, you have a problem. I would start by checking all the check valves.

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[simp5782]

Start by making sure all your lines are hooked up right between your spring brake cans and the tree on top of the axle. People sometimes hook air lines up wrong. If you need a diagram pm me and i will email it

Also need to make sure all the rear brake supply lines are right as well. Trucks 3ports on their spring brake chambers are hooked up to different ports than a 2 port brake chamber.

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[Speedzilla]

Hey Guys,

Been checking pressures at the cans this evening, and I have a question, in the troubleshooting TM it states:

"Air pressure to service brake chambers must compare with reading on primary air pressure gauge. Air pressure to spring and service brake chambers must compare with reading on secondary air pressure gauge."

My service brake chambers (front, and single service brake chambers in the rear) seem to match the primary tank air pressure, but the service/spring combination chambers seem to match the primary air tank pressure as well. If I hit the pedal a few times, primary pressure (and pressure reading) will drop to around 90PSI and the secondary 115 or so. At that point the service/spring pressure drops to 90PSI to match the primary tank. And this is where I am confused.

I am looking through the P2P and from what it looks like, the secondary tank only operates the front service brakes, not the rears. But the TM has the above quoted line in the rear troubleshooting.

Anyone have any insight on this?

 

[simp5782]

The secondary tank runs to the Q2 valve on the front of the truck then goes to the front brakes.

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