air-pack principles & diagnosis

[808pants]

I've just pulled apart the second of my two air-packs, both of which failed in that 'where'd my brake fluid GO' way (it was all in the air-cylinder, naturally). I'm not seeing any deals on ebay or elsewhere, and I'm kinda fed up with just putting these piles of parts aside for later anyway, so I'd like to rebuild at least one of them with some confidence that it's not going to do the same thing in short order.

I should already have disclosed that mine's a 5-ton unit - but there doesn't seem to be as much discussion of them over in the 5-ton area. While casting about for solutions, I came across the (reattached) section-drawing of the deuce-type air-pack, and it looks to me like the operation is identical even if the components don't match. So what I am trying to understand is how all that fluid actually gets into the air-cylinder portion of the unit when they go south. I don't think a scored or pitted slave cylinder will do it - that would just let the pedal drift to the floor when it gets really bad, right?

There appear to be two routes:
1) through the armada of seals (on the 5-ton, anyway) that surround the air-cylinder piston-shaft, and
2) via the small "relay piston," through its back-to-back seals or via damage to the bore itself.
Are there other routes?

Again, I don't know the specifics of the internals in the Deuce version of the air-pack, but in both of my failed 5-ton units, I didn't see any obvious bore or seal problems in those locations - (though the slave seals were atrocious on the oldest one, that shouldn't have caused fluid to get into the air-pack, I think.)

If I knew for sure that the problem lies within the relay piston/ or its bore, I would send that housing off to be sleeved, then rebuild the rest after just cleaning things up with a hone and some brakleen. Somewhere else to suspect? Anyone have experience along these lines?

 

[808pants]

adding attachment...if lucky...

 

[rlwm211]

From what you have explained it appears that the main piston cups failed, most likely due to age. In the old days, before power brakes, a master would fail and then you had brake fluid inside the cab. It would leak out past the seal and out of the bore.

In the air pack, since the air piston is what applies the force to push the hydraulic piston the only place the brake fluid can go if the hydraulic seals fail is back into the air piston and actually behind the piston. That is where the bore leads to.

Rebuilding these requires attention to detail and uncompromising care to surfaces and bores in order to ensure a long lasting rebuild. Kits are available from several national suppliers. The instructions are in the TM's and are pretty comphrehensive. You will need Silicone brake fluid, OHT (Oil Hydraulic Tool) and some time and whatever tools that re required.

Compared to buying a new, or rebuilt air pack, you can save a ton of money.

Hope this helps.
RL

 

[808pants]

[QUOTE
...In the air pack, since the air piston is what applies the force to push the hydraulic piston the only place the brake fluid can go if the hydraulic seals fail is back into the air piston and actually behind the piston. That is where the bore leads to.
...
RL[/QUOTE]

RL - thanks for your reply. Here's the thing - from staring at the components and looking at the attached drawing, a leak in the seals at "P" (which I think is actually called the slave cylinder in this configuration) would not actually cause a "leak" of fluid, other than into the "pilot-pressure" side of those seals (where there's still fluid, but under pressure only from the master cyl). This I think would result in slippage at the pedal, that is, the pedal would slowly drift to the floor during braking, getting worse as the seals continue to wear - but otherwise giving decent braking operation, and with no drop in reservoir fluid level. Check out the section drawing - the air-cylinder piston pushrod, which is doing that pushing, is still sealing off the air-driven components from fluid since it passes through a series of fixed seal/s (under the big end of the air spring). And in my two disassembled cases, I've seen nothing suspect about those seals nor the shiny pushrod shaft. Those pushrod seals should have an easy life, I surmise, since they don't ride against a big bore nor need sharp lips like the cup-seals do - so they should far outlast cup seals, IMHO. This causes me to look elsewhere.

I had components of one air-pack nicely cleaned up, and was in the process of using a rebuild kit to put things right when I discovered that the pitting in the slave bore was kinda bad. As per your conclusion, I thought this might doom me to reliving the same problem (fluid in the air-cyl) since the pits are too deep to hone out. (Getting quotes for sleeving, etc...yuck.)

But while sick for a few days it was nagging at me that the operating principles of the thing I was trying to diagnose and rebuild were not at hand, so I couldn't really properly analyze. I found that someone else posted the attached, (and found an analogous 5-ton drawing) and I've been staring at both to see by what other paths that fluid can get into the air cylinder.

I'm more suspicious, really, of the "relay piston" and bore (leakage through there would put fluid right into the air-cyl, and it's a much bigger diameter, so I suspect more of a challenge to keep sealed). Still, I wanted to see if I am overlooking something.

I'm leaning towards honing that relay-piston bore a bit, and leaving the pitted slave bore about as it is (no more honing), reassembling, and maybe cobbling up some method of bench-testing, since I have the master out for rebuild as well.

Someone mentioned having installed an air-filter on the compressed-air line (no hardware details given, though...anyone?) and I think that would be an excellent (maybe vital) mod to make. Keeping air-system grit and maybe some water out of the bores seems like a no-brainer choice. Wondering about air-tool line filters...and maybe even an air-tool oiler. Otherwise, it's wet, dirty compressed air against a haphazard and rare shot of ATF (thru the plugged port) to try to keep that whole bare-steel air cylinder free of corrosion. I'm surprised they don't go bad more often, really.