M818 Brakes locking up

[kstate90]

One of our 5 ton fire trucks had the brakes lock up on it yesterday. We picked up an air pack to install today but had a few questions before we install it. They drove it tonight and it only locked up once but released when they backed up to return to the station. Last night all four wheels were hot so I don't think it is one brake causing the problem. He said the pedal feels like it hits an air pocket or dead spot when you push on it and then it goes down about and inch or so and you can fill back pressure. Is there something other than the air pack? Master cylinder? Thanks.

 

[R Racing]

It could be the master cylinder. Or maybe even air in the system.

 

[doghead]

Pushing back sounds like airpack. I would rebuild the Master cylinder, while you at it.

 

[Tow4]

Check adjustment on the brake pedal. It needs to have a little free play just like the deuce.

Ray

 

[kstate90]

I went and looked at the truck today. I started it up and pumped the brake pedal a few times and the brakes were locked just sitting in the station. I disconnected the glad hand on the back but it did nothing. From the feel of the pedal I think it is the master cylinder not venting back. I could hear the air pack venting but the pedal did not return right. We are going to take into town Monday and have the local mechanic check it over and make sure everything is right. Brakes are kind of important.

 

[jwaller]

sticking Mater cylinder. do not drive it into town, have it fixed where it sits. when those drums overheat you might damage the tires, seals in the drums and burn the truck to the ground or run over somebody when the brakes fail.

 

[cranetruck]

Glad hand cover need to be vented or a small leak will build pressure and lock brakes...

 

[jatonka]

Cranetruck means the front gladhand cover on the right front corner. It has a breather or the newest ones just have a pee hole to vent the system. It has cost some of us a lot of money replacing parts to find the breather was plugged up. I learned the hard way. Now I have an extra MC, airpac, and all the rubber brake lines . JT

 

[kstate90]

I looked at the glad hand on the front and it did not look like it was blocked. I had read that in one of the other posts. I am going to call the mechanic this morning and see if he can work it in today. We dumped the water off last night so it would be that much lighter. I will post what we find out today. Thanks.

 

[m816]

lOCKED UP BRAKES

jUST LIKE JTONKA SAID, CHECK THE RIGHT FRONT GLAD HAND. THAT IS WHERE THE AIR SYSTEM RELEASES AIR WHEN YOU REMOVE YOUR FOOT FROM THE BRAKE. IT SOME TIMES LOOKS CLEAR BUT STUFF AND CRITTERS HAVE A WAY OF CLOGGING IT UP. JUST REMOVE IT ALTOGETHER AND CHECK INSIDE THE ORIFICE TO SEE IF IT IS CLEAR. JOHN IS CORRECT, IT HAS CAUSED A LOT OF US TO REPLACE PARTS THAT DON'T NEED TO BE REPLACED AND IT WILL DRIVE YOU NUTS UNTIL YOU FIGURE IT OUT.WORSE CASE COULD BE AND AIR PAC BUT THAT IS NOT A REAL HARD JOB EITHER. MAKE SURE THE FLUID IS REPLACED WITH SILICONE FLUID. WE FOUND THAT AFTER REPLACING THE AIRPAC AND MC THE WE FINALLY HAD TO PULL THE WHEELS AND REPLACE THE WHEEL CYLINDERS CAUSE THEY HADD RUST AND SLUDGE IN THEM FROM SITTING FOR SUCH A LONG PERIOD. THE SYSTEM WOULD APPLY THEM BUT THE RETURN SPRINGS COULD NOT RELEASE THEM BECAUSE OF ALL THE GUNK INSIDE. REMEMBER WATER AND SILICONE DON'T MIX SO IT SITS AT THE BOTTOME OF THE BRAKE CYLINDERS UNTIL IT PREVENTS THEM FROM WORKING. THAT HAS BEEN THE CASE ON MY LAST THREE FIVE TONS. I 'M GETTING GOOD AT IT NOW. YOU NEED HEAVY JACK STANDS AND A WHEEL DOLLY AND A STRONG BACK TO DO IT BUT IS REALLY ONLY A COUPLE DAY JOB.

 

[Ferroequinologist]

I had the exact same symptoms on my XM818- I had rebuilt the MC, and a few months later the brakes were locking up on me on the way to Charlotte for the Memorial day display. It was a mis-adjusted brake pedal. The rod was too tight and not allowing the piston to return all the way in the MC to vent the pressure, so everytime you applied the brakes they got tighter and tighter.

Very easy to check, much less fix. If that isn't it, I would also check for a sticking MC.

Like Jwaller said, they will get hot in a hurry! Fix it where it sits.

 

[kstate90]

Yesterday we drove the 5ton into town to the mechanic. Brakes were unlocked and we drove the truck using the transmission to slow up for corners. They looked the truck over from front to back and ended up taking on of the rear duals apart to determine what was going on. After inspecting the brakes they determined that the brakes were out of adjustment and that the wheel cylinders were fully extending to apply the brakes. They adjusted all of the brakes and test drove the truck. It now has brakes that will stop the truck on a dime. The chief drove it back to the station and he said the brakes were very good on it. They could find nothing else wrong with and could not get the brakes to lock up after they adjusted them. I will update the thread if it acts up again. Thanks for the help.