How many brake applications on a full reservoir?

[Old_DM]

I have a 1970 DM685 dump truck which seems to run through it's air much faster than a modern truck.  With the engine off and air pressure sitting at 130 PSI I can make 4 full pressure brake applications before the pressure drops to 65 PSI and the air warning turns on.

There are 2 tanks in the system and all 4 segments drop to the same pressure, so I know it's using the full reservoir capacity.  No leaks in the system, truck stops as it should when the brakes are used, no air leaking when the brakes are fully applied.  Pressure builds from 0 to 130 in under 3 minutes when cold started.

Total of 6 air chambers, not including the 2 parking / spring brakes.

Is this just normal for a truck of this era?

[theakerstwo]

make sure the brakes are adjusted up.

[Old_DM]

First thing I did.  Front brakes had excessive slack, but after adjustment to minimum, no change.  The rears are so tight you can barely see any motion when the brakes are applied.  Parking spring brakes work as they should.

[Joey Mack]

is there a lot of water in the tanks, that is taking up room for air?  and to the best of my memory a typical brake application uses 15-20 PSI.  so 4 applications will be 60-80 PSI of usage..  are all of the rear brake chamber's 30's?  (30 cubic inches) ...  jojo

[Onyx610]

Have you listened for any big leaks when making a brake application?

[mechohaulic]

when you say " all 4 segments drop the same" , does this unit have dual air system ( red and green needle on gauge);i don't remember when that started. when dump body was installed, were the air tanks moved or replaced? the main wet tank has a check valve connecting other half of tank;; when gauges are at 130 if you open each drain on the tanks individually ; does each drain have full pressure of air coming out ?  have seen the one way  check limit air to other tank half. tank not having an external check has one internal , pain in a-- to change,  AFTER draining tank remove fitting on end of tank flashlight can see the internal check,  long extention and grease on the socket will remove the check. have also seen an improper discharge line from compressor  become restricted  from wrong line too close to high heat compressor air.  also as you stated ;;yes older units weren't made to build air like todays iron ;; NOT counting fire apparatus ; they had a rapid build up system. 

[mechohaulic]

do you have an air dryer that hasn't had the filter changed  restricting air flow ???  being a '70 doesn't mean someone hasn't "modified" things through the yrs .  has this issue been there or just become noticeable ?? 

[Onyx610]

You can check your “check valves” by draining the wet tank. If primary and secondary hold they are good. If one of them drains down that one is bad. 

Edited September 6, 2022 by Onyx610
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[Old_DM]

No air drier in the system.  All tanks have been drained, no excess water in any of them.  The wet tank section tends to accumulate oil, but the air builds from a cold start to full pressure within the specification that the Mack service manual from that era indicates as OK.

Check valves in each tank seem to work as they should.  My first thought was that someone plumbed the air exit to the wrong tank segment and the system was being fed from only one tank.  Testing has shown that is not the case, as all 4 tank segments fall to the same pressure when brake applications are made.

The system has 2 tanks, with 2 drains per tank.  The truck has been seriously neglected by previous owner(s) and the air piping is quite a circus.  Reading all the comments, I suspect it might just be normal or have oversized pots, since it stops with minimal brake application when loaded to the top with dirt.  Makes for a real fun time when trying to make K turns on a grade...  It's off road only at this point.  Thanks for all the input.

[mechohaulic]

thanks right back to you for the latest update.it does become  frustrating when questions are answered by many on this forum  and the "questioner" just vanishes from the planet, LOL . 

[Joey Mack]

Updates are great...  

[fjh]

Yup good on you for following up!

[Onyx610]

10 minutes ago, fjh said:

Yup good on you for following up!

F, look at the date bro…🤭

[fjh]

Better late than never bud! Thanks 🙏 

[Onyx610]

Ok I’ll go with that haha

[Onyx610]

Im actually surprised at how few air brake questions this site gets.  

[JoeH]

He probably has big brakes. Our 1980 dm686sx has 18"? Brakes instead of the standard 16"? Brakes.  Truck always had great stopping power. I wish more trucks had them.

[mechohaulic]

larger brake shoes a positive on vehicles especially dumps . back to original question of air consumption  per brake application; based on everything written , i would go with a third tank or larger if room is there. shouldn't have low air buzzer with normal stopping every time. did a reread . this air drop is with engine off not driving. its seems ok then. doesn't state  air loss when running. have  a great day😄

[mowerman]

Seems like more like 8 to 10 times before our alarm goes off but that’s not mashing it to the floor and yes, it’s an old thread

[Old_DM]

Yea, I can get more than 4 presses if I don't mash it to the floor.  Think that was part of my issue.  Didn't consider that more pressure on the pedal equals more air drained from the tanks and may not be needed when just holding the truck.

Can someone explain to me how the brake hold feature that is on the steering column is suppose to work / be plumbed?  It was disconnected when I got the truck.  I assume that this lever is suppose to provide pilot air to a valve somewhere in the truck that basically cuts off the vent route from the brake pedal when it's released, thus holding the brakes in the applied position even after you let up.  Or does the lever actually apply the brakes like the treadle?

Thanks again for the input.

[Geoff Weeks]

The trolly valve/Johnson bar, is normally plumbed to apply the trailer brakes only.  It is independent of the treadle valve, both come together at the double check valve ahead of the tractor protection valve. Whichever pressure is higher gets sent to the trailers blue line.

[Old_DM]

This is on a dump truck, though it has a pintle so I suppose it might have been for use with a trailer.  I assumed, perhaps incorrectly that it was used for holding the truck stationary when dumping vs the parking brake. 

[mechohaulic]

does the unit have two glad hands at the back by the pintle?? dump having tag trailer capability use the trolly same as a tractor uses for trailer . trolly is a hand version of treadle valve. on dumps not having trailer/air brakes for tag alongs; the trolley can be hooked to rear wheels of the dump truck itself.

[Old_DM]

There are tons of parts lost / missing / disconnected so it's unclear what the original configuration was.  I have a copy of original build sheet but most of it is hand written and microfiched so a fair bit is unreadable as wall as having been replaced over the years like the xmission. 

There are no rear air connections for a trailer at this time.  As the PTO requires the transmission to be in gear with the compound in neutral, I was going to repurpose the trolley valve as an alternate way of holding the brakes while dumping on an incline vs constantly hitting the parking brake.