Old_DM

Thanks. I'll be sure and use some purple Loctite on the holder.

My truck started loosing prime before I got to changing the filters. It had mysteriously died on me a couple of times, only to start again with 30-60 seconds of cranking. Finally gave it up after sitting for an hour shut down after a days use. Luckily I remembered this thread and cleaned up the hand primer, pumped it for a minute until there was noticeable resistance and the truck started right up. The hand primer leaked a bunch on the first few pumps but then sealed. Parts book doesn't show the lift pump / primer as having any internal parts available. Looks like PAI makes replacements for both the pump and primer. Anyone have experience with them? The parts book also shows no gasket between the lift pump and the injection pump housing. That can't possibly be right. Or do they expect you to slobber it with some sort of sealant?

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.

Thanks for the info. Love the air tank pressurizer, great idea. Hopefully I won't have to go that route. On double fuel tank set ups, is there a check valve between the main tank and the feed from the aux tank? I have a weird looking right angle fitting poking out of the aux tank before the line that connects over to the main tank. Are these set up to only allow draining from the aux tank towards the main?

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.

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.

Have put off changing the fuel filters for way to long on my DM685 (1971) due to worries about getting it primed. It was well used and abused when I got it for cheap, but the fuel lines all look original and correctly plumbed. Suction through the coarse filter, into the lift pump / primer then out to the fine filter and into the injection pump. My question is, when I remove the filters, will the main injection pump wind up with air in it? Especially after removing the fine filter. I plan on filling the new filters before installation. Will I be looking at having to loosen the injection line couplings at the injectors to purge air from the lines or is the primer on the lift pump used to purge air through the top of the main injection pump housing? Also, are the old factory fuel fittings all 45 degree flares on these old trucks, or some long abandoned standard?

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.

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.

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?