brakes locked up

[davesgmc]

I already read all of the other posts, but I still ahve to toss this out there for ya'll to give me advice on. I ahve my elmo truck, a 5ton brige truck, 139 series, the brakes are locking up. it strted more than a year ago, you apply the bralkes and they seem to stick sometimes, wait a few sec and they release. i did learn if i sit and pump the pedal to hear that air release before i go driving off they seemed to work ok. well i just been dealing with that issue for a while now, was no big deal to me.

but now thay are locked up solid. If i apply air to the system via my shop compressor ot the truck air either the brakes apply and lock up. when i drain the air, they release. yes, with no air i can see the return port returning fluid so thats not the issue. io can actauly see the brake master cyl pison move when air pressure is applied. i also tried to apply air pressure top the right front glad hand and it moves real fast and hard. just like its supposed to. but when air is put in the truck system, the master cyl pison moves, i can see it through the top of the master cyl,,and the brakes apply. I already flushed the complete sytem and put in new fluid.

is my air pack fianlly gone bad? I had been oiling i every now and then, and even tried to oil it a bit more now with no results. when you take the center plug out of the back of the air pack and put air to the system air does coem out of it. is it suppoed to do that? is there a quick fix for this that i can try or do to get it to the show this weekend? or am i looking at rebuilding the air pack? what is in a rebuild kit? do i relaly ned one? or can it sually be taken apart, cleaned nd put back?

advice please?

 

[militarysteel]

i'd take the main air line off, the one going into the airpac... and squirt while lith grease up the hose and into the air pack. then bleed down the hydraluic pressure at the top of the airpac, from the master.

i had the same issue.. its all better now.

 

[davesgmc]

there is plenty of lube in the air system of the air pac for sure. i lubed it from the rear and the supply line both. its like the air pressure is getting into the part of the air pack that is actually applying the brakes. havent got the unit off yet as i still ahve too much other stuff to do. if there is no quick fix for this it will just have to wait.

i am still assuming its the air pack thats bad?

 

[jwaller]

sounds like an airpack issue. try lubing it and see if that works. if not you need to rebuild it. the kits run about 150-190 for them.

 

[davesgmc]

is the air pack rebuild kit for a 800 series the same as the 139?

 

[Chevytruck]

You check the vents?

 

[davesgmc]

yup, venbt free and clear, just wont vent. it used to venbt only when pushing really hard on the brakes to get the air pack to free up and start working, now it wont release air at all.

 

[davesgmc]

follow up to this story,,,took the air pack out of elmo, took it apart,,,really didnt see anyhitng wrong with it, pretty clean inside, no rust, water, etc. seemed to look fine. went ahead and replaced all the seals, wahers, valve, etc,,stuff that came in kit i bought off ebay for $24.99. the only seal i didnt replace was the main seal in the large chamber,,, by the time i ogt to that, i was ready top put this thing back in so i decided not to mess with it.

its in, brakes bled, and they work GREAT now, they release every time, and stop great.

problem solved

 

[Irv]

That happened to me, too. I took the airpack apart and it was just gummed up. All the seals were fine, so I put it back together after cleaning and it was perfect after that.

Now for the field fix: I didn't know it at the time, but the fix for that is to slightly crack open the valve on the glad hand on the front of the truck, so it just barely hisses. The pressure that you applied to the control piston was unable to release normally. The cracked open valve up front would have accomplished that. Something to remember if you are stuck somewhere, like I was. Irv

 

[davesgmc]

well brakes WERE working fine. went out this am to warm it up to go to the parade this morning. pedal went straight to the floor! i saw there was a bit of a leak at the right middle wheel,,assume wheel cyl started leaking. fluid in master was completely gone, i went and got a few more gallons of fluid and bled the brakes/ lots of air in system naturally, bled it all out, but didnt seem that it had any braks at all, pedal pumped up, but no power assist at all this time, like somehting is stuck. oh well, back to the drawing board.

 

[808pants]

[QUOTE...the only seal i didnt replace was the main seal in the large chamber...[/QUOTE]

I'm slowly wrestling with a few of these air-packs now. To me it's a complicated system that requires a lot of analysis (staring at parts, the section drawing, checking passages to see where they go, etc) since I can't find any existing documentation that adequately explains their operation, but I think I've got it pretty well figured out now.

Did you open the air-cylinder side at all, then? On two of mine I have found that's the secret graveyard of all that expensive new DOT5 that went missing. I've still not finished rebuilding them owing to other distractions.

If that does turn out to be where the fluid went, it still may be hard to precisely determine what route it took in getting there - though if you only rebuilt the rest of the unit and left the air-side intact, I think that also means you couldn't have changed the seals that keep fluid from passing along the 3/8"-ish air-piston pushrod and into the air-cylinder itself. (I had a bitch of a time getting the bottom-most seal out of the last one of these I was working on - it's press-fit into the aluminum housing, and almost blind from the other side, so it can't be driven out - book says to thread a 1/2" NC bolt into it and then use that as a perch to drive it out, but mine was far too tight to give it up that way, so it came out in many little pieces instead...)

The other fluid-to-air seals are the lip-seals in the little "relay piston" ("C" in the attached section dwg). I'm finding I'll have to hone that little cylinder nearly to death in order to get past some sloppy-rebuild-scratches from the last guy.

I marked up the attached section drawing - looks to me like it's got an error or two in the labels originally used. The air-pack rebuilding instructions are also attached in case you've not already got them.