CAT 140H strange parking brake issue

[RandyAndy1982]

Evening, folks, I’m new here so take it easy on me.

I’m a long-time truck mechanic, starting to venture into the equipment world. Things have been going pretty well, but I recently took on a job that’s got me stumped.

The machine is a Cat 140H, I believe about a 1997 (PIN 2ZK02124) with parking brake issues.

When you start the machine and let it build air, everything seems fine, though the air builds all the way up to about 150psi. The low air alarm doesn’t turn off until ~90psi. That’s a bit different than I’m used to from the truck world, so I want to verify that it’s correct.

At this point you can release the park brake and everything is fine - however, if you happen to use the brakes or somehow drop system pressure to around 115psi, an air leak “appears” out of thin air, coming from the transmission vent. Once this starts, that side of the tank will be empty in about a minute, maybe 90 seconds with the engine off/key on.
HOWEVER - if you start the engine and build pressure back up to 120 or so, the air leak will stop and it will hold pressure even if you shut the engine off again. Step on the brake (or vent some pressure out of the tank drain) and hit 115 again, and the air leak reappears and drains the tank.

During the time that it’s leaking, if you drop the lever back into Park, the leak will stop. If you turn the key off, it stops. Turn the key on and take it out of park below 115psi, the leak starts again.

Pressurize the system above 120 with an outside air source, and the leak stops.

During the leak, the park brake starts out released, but the pressure falls so rapidly that it applies within 20 seconds or so.

It’s the damnedest thing. I’m 90% sure it’s got bad seals/o-rings in the parking brake assembly - I mean, what else can it be? The other thing is, it’s dead-nuts consistent. Does it every single time you meet the parameters.

I’ve checked the whole system from where the parking brake air supply tees into the right-side supply line from the tank to the service brake valve, the check valve, ignition-controlled park brake air solenoid, the park brake control valve in the shift console, and the hose to the park brake from that spot under the right side of the cab. All of these components seem to be normal.

So am I right in thinking the park brake seals are shot and just happen to be leaking in a certain pressure zone?
And what pressure should the governor be kicking out at - or is 150 or so normal?

Thanks in advance, everyone, and thanks for letting me join the forum. Good night!

Randy

 

[GEORGIOS198217]

Mr, you check the air dryer valve?

 

[Nige]

I'm not going to reply to each point on your post above. Attached is the SysOp and T&A for the air system. I didn't include any specifications because they are all separate document but if you need any specific ones then please ask.

A few random comments: -
As per George's question. According to the "as built" documentation it is equipped with an air dryer. Has it been serviced recently.? This would be my first stop.
Take a goos look at all the drain points and open valves.pull plugs to check for the presence of water. That will tell you a lot.
Compressor should cut out at around 140psi and cut back in at around 115psi.

 

[RandyAndy1982]

Thanks folks - the township’s records indicate that the air dryer was serviced two years ago, but whoever noted it did not note the hours on the unit at that time. I’ll be back out there tomorrow and will pull the dryer apart.

All drain points are functional and had no moisture when I opened them. Pulled the outer & inner plugs on the parking brake itself and released the parking brake to blow full air pressure through it - got a small amount of water & sludge out but not as much as I expected. I did get about a pint of oil when I removed the outer plug, think I read that was to be expected.

Also noticed yesterday that the “low air” alarm buzzer & light come on at 120psi - seems a bit on the high side for a 140-115 range. Verified the governor is a 140psi unit.

Thanks for the pdfs, I’ll give them a look this evening and see what I missed.

 

[Nige]

If the air dryer was last serviced 2 years ago it ought to be done again. TBH irrespective of hours a dryer really should be serviced every year. Less operating hours per year are actually worse than more when it comes to cycling the dryer.

 

[RandyAndy1982]

Makes sense to me - we do dryers on all the trucks in our municipal fleet annually just to prevent any moisture from freezing up while they’re out in our -25°f winter temps.

I’m going to get out to their garage tonight and pull it apart to see what I can see; I already called in the parts order for it. Hopefully it helps, but it sure won’t hurt.

Thanks!

 

[Mobiltech]

If you have air going into the trans and coming out the breather you definitely have park brake seals leaking. It’s not that hard to reseal.
I think it takes nearly full pressure to completely release it. It may be that the piston is cocking a bit in the bore as it starts to come on causing more leakage.

 

[RandyAndy1982]

You know, I never considered the possibility of the piston going crooked… that’s a better explanation for the odd symptom of sealing fine above 115psi and leaking thru the trans breather below 115 than anything I’ve been able to come up with.

I already have all the seals, I was just waiting until I verified everything else (and until they finished grading their last 2 roads). I’m going to service the air dryer, then check the governor, then pull the park brake assembly off to see what I can see.

Thanks again for all the advice!

 

[Nige]

Here's the R&I for the park brake if you plan on going into it.

 

[RandyAndy1982]

Incredible! Thank you so much, this is far beyond what I had hoped to gain here.

 

[Nige]

Incredible! Thank you so much, this is far beyond what I had hoped to gain here.

Don't forget to come back and give some feedback on the problem & the fix. Also post photos - the forum likes photos.......

 

[RandyAndy1982]

I absolutely will. I’m working around my day job and their shop availability out at their town garage, plus my work at my home shop, so it’s a bit hectic. But so far it looks like I’ll be out there this weekend to pull it all apart, that way I can order any hard parts I may need on Monday morning.

 

[Nige]

Here's the parts list for the park brake.

 

[RandyAndy1982]

Fantastic!

 

[Nige]

I would suggest that you always confirm Part Numbers when ordering based on your machine S/N. The dealer parts dept may have access to newer information.

 

[RandyAndy1982]

I’m out at the town shop today working on pulling the park brake assembly. I pulled the aerofiner apart and it’s pretty dirty inside. I’ll get a new cartridge for it this week.

Once I get the brake assembly on my bench at home, I’ll post some pictures.

Thanks again, folks. Didn’t want you to think I’d forgotten to update this thread.

 

[Nige]

I pulled the aerofiner apart and it’s pretty dirty inside.

Told you they need replacing every year without fail if the machine is not doing a lot of hours..........

 

[RandyAndy1982]

Didn’t doubt you one bit, and not at all surprised by what I found in there.

Beyond that… got the parking brake assembly home and on the bench. The inner piston (the smaller disc) lifted right out with zero drag at all. Set it back in and it just ‘plops’ right to the bottom. All of the o-rings feel hard as rock (no surprise, I guess).

Looking for damage or wear, the only thing I spotted right off the bat is what appear to be drag marks from the inner piston rubbing on the inside diameter of the outer piston (I wish they didn’t call them both ‘piston’).

I’ll take a closer look tomorrow in the light of day, but I’ll post the pictures I have now.

Thanks folks, and g’night.

 

[Nige]

See if the arrangement of parking brake shown in these specs match up to what you have.
Especailly check things like free length of the springs.

 

[RandyAndy1982]

Hey folks, reassembled all last night and everything works as it should… so the low air alarm is the last thing I need to address.

I’m guessing it’s a failing air pressure switch. It doesn’t turn the low air alarm off until it’s well over 120psi, and I believe that it should be around 77psi based on the manual specs.

I was wondering if anyone here could help me out with a part number for it? I can’t seem to find it in our parts lookup guide.

Thanks in advance!