R Model Air start losing air over two days

[thebaz]

I think 2-3 weeks.

I could start the truck after months without a problem for it's whole life until recently

[41chevy]

1 hour ago, thebaz said:

I think 2-3 weeks.

I could start the truck after months without a problem for it's whole life until recently

Rust under the tank straps.

[thebaz]

Quote

Rust under the tank straps.

I will check the tank physically again, but I did check to see if there was any leaks from the tank and fittings with soapy water.

My suspicions are that I may have missed a defect in the tank somewhere that is leaking, or that there is a tiny pinhole in the airline going to the start solenoid on the firewall. I did check the solenoid for leaks and may have to re-visit that one again as well. The truck did hold air and start strong after about four weeks the previous time I started it so it was a definite improvement unless there is an intermittent leak and it was just coincidence that it seemed better after all the work on the valves etc.

I am thinking short term i will run a new airline to the solenoid with a gauge to check the pressure loss and a tap to isolate the solenoid so I will know if it to do with the tank and start valve or the electric solenoid end.

[thebaz]

After around 18 months of holding air between starts the Mack is suddenly back to it's old trick.  I have searched like crazy with soapy water and found nothing. I removed the tank thinking that the corrosion between the straps and the tank may have created a leak and after testing the tank in a wheel barrow full of water could not find one bubble leaking out. I fitted a pressure gauge to see how much air is in the tank and a tap to isolate the start solenoid on the firewall. With the tap off it dropped from 125psi to 60psi in three days. With the tank on 125psi I removed it from the truck to test it and found nothing. the pressure actually went up slightly from the heat of the sun. It has not dropped at all in six hours. Although I have replaced the no return valves, I am suspicious that somehow there is still some ability to equalize the pressure between the wet tank and the start tank as the air in the truck seems in sync with the start tank. With the non return valves submerged under water there was no trace of leakage at all. there is a valve on the wet tank that feeds to the start tank, I am attaching a photo of the valve. Does anyone know what it is?

I am going to leave the feed line from the wet tank to the start tank off so there is only the pressure from the start tank pushing against the non return valves and see if it hold air over a few days.

Not sure why it seems to have no leaks off the truck and holds air at least over the six hour period, but hopefully a process of elimination will yield something.

If anyone has any ideas please chime in.

[41chevy]

Take a close look at you air horns.  Had a leak  that would come and go with no pattern and no visible leaks with soap check, turns out the air horn valve would stick a few thousands and leak down the system.

[mrsmackpaul]

Not sure how the US Macks are plumbed 

In Australia the most common cause of what describe is two non return valves on the T on the air tank

Or the solenoid valve

Or a rubbed thru air line in the start system 

Gotta remember there is only 4 valves a tank and two air lines on the setups that I have had anything to do with

Paul 

[thebaz]

Thanks for your reply. The air horns should not effect the start system as it is supposed to be prevented from having the rest of the air system from bleeding air from it. I have had problems with the horns robbing air from the air system though.

[thebaz]

14 hours ago, mrsmackpaul said:

Not sure how the US Macks are plumbed 

In Australia the most common cause of what describe is two non return valves on the T on the air tank

Or the solenoid valve

Or a rubbed thru air line in the start system 

Gotta remember there is only 4 valves a tank and two air lines on the setups that I have had anything to do with

Paul 

This is an Australian Mack and I have replaced both non return valves and the whole T setup. There is no air escaping from anywhere that I can see when the tank and it's valve are submerged in water when removed from the truck. The main solenoid valve is also not leaking  and was rebuilt 18 months ago. I have a lever valve that close air from the airline to the electric solenoid so that rules out any wearing through the air line or leaks at the firewall valves. Nothing should be wrong here, but it is.

[mrsmackpaul]

Can you pull the air line off that supplies the air to the tank before it reaches the non return valve and put it in a bucket of water and and look for leaks

I also pour water into the air line fitting that you connect a air line to refill the tank as most times it points upwards

Check the O rings on the main big air solenoid (you can unscrew the cap on the bottom)

I lube it up with vasoline as I dont think it perishes the rubber 

Apart from that unbolt things and put the whole tank and fittings in a stock trough or borrow someones swimming pool and you can check the whole system I guess

 

Paul 

Edited November 28, 2018 by mrsmackpaul

[Freightrain]

My guess that is a check valve to feed the start tank.  That would be the first thing I would check/change to see if you start holding air better.  I know a few times I've lost air quickly on my start tank(over a week).  Only thing I can think was some piece of crud got stuck in the check valve( I use Sealco brand).  They are new, but if something gets stuck it will bleed off.

I've heard mentioned before about dropping the air in the main system by hitting the brakes a couple times after shutting motor off.  That will drop the air pressure on the wet tank, seating the check valve better then waiting for it to slowly drop off.

[thebaz]

There is no leak from the supply hose to the start tank and no air exiting from the non return valve on tank when the airline is disconnected. There is no air leaking from the start solenoid on the tank and I have submerged the tank in water so I know there is no leak at the solenoid or non return valves. The air in the start tank has only dropped maybe 20-30kpa over 24hrs, but the brake system is nearly out of air where as they were in sync over the past week.

I have no idea at this stage how the air is getting past the non return valve when it has pressure on both sides but that seems to be the case. Nothing that I can see has changed and yet it was working fine for the past 18 months.

[thebaz]

6 hours ago, Freightrain said:

 

I've heard mentioned before about dropping the air in the main system by hitting the brakes a couple times after shutting motor off.  That will drop the air pressure on the wet tank, seating the check valve better then waiting for it to slowly drop off.

I think you may be on to something with that procedure which is exactly what I was thinking of trying as I always wait for the compressor to shut off before I stop the motor at night and maybe the equalization of pressure allows the non return valve to be ineffective. It is definitely working 100% with pressure only from one side.

 

[thebaz]

I think I have found what the valve is. It seems to be a "Pressure protection valve". I think it's job is to not deliver any air until it reaches the set pressure and will also close below a set pressure.

I am thinking maybe it's main function is to allow the air to build up to say 80psi in the brake system before sending air to the start tank which would allow faster charging of the brake circuit to enable earlier spring brake release.

it should also close when the wet tank drops below pressure and theoretically retain the air in the start tank.

Anyone confirm this?

[thebaz]

After the truck sitting for over 30hr the air pressure has not dropped at all. i am pretty sure the Pressure protection valve will rectify the problem now. I have started the truck and after building up full air, I have pumped the brakes to drop the pressure on the supply side of the non return valve, ( as per freightrain's suggestion), and see how it goes in the morning.

[mrsmackpaul]

21 hours ago, thebaz said:

I think I have found what the valve is. It seems to be a "Pressure protection valve". I think it's job is to not deliver any air until it reaches the set pressure and will also close below a set pressure.

I am thinking maybe it's main function is to allow the air to build up to say 80psi in the brake system before sending air to the start tank which would allow faster charging of the brake circuit to enable earlier spring brake release.

it should also close when the wet tank drops below pressure and theoretically retain the air in the start tank.

Anyone confirm this?

Yes 

The last bit of the air system to be charged should be the air start tank

This is my understanding and every Mack I have had anything to do with is the same

 

Paul 

[Freightrain]

Interesting thought about that valve.  Sounds plausible.  I think the Sealco check valves I use have a break open point like the one you show.  They might not be as high as 80#?  I'd have to see the specs on them.  I know my truck doesn't take long to fill the two tanks I run on the main system, then it takes much longer to get the air start tank up to 120#.  I can hear the check valve squeak as it works on the last few pounds.

[thebaz]

this is the valve I think I will replace it with,

http://www.sealcocvp.com/prod/catalog/140200/index.htm

It has a non return valve built in as well. Should cure the problem hopefully. I am still a little puzzled as to how the air started to pas through the non return valve as it is new and does not leak at all with pressure only on the start tank side.

Edited November 30, 2018 by thebaz

[mrsmackpaul]

I think something people dont realize today is that when all these trucks were getting used everyday for a living this was rarely a problem as the trucks rarely stopped for longer than 8 hours, so as long as the air held over night all was good

On the odd occasion I have gone to start the truck after it has sat for a month or so during the wet season and it held air and other times they only just have enough air left over night to swing into action

I have often wondered why there is no gauge to monitor the air tank, that way you could see if a problem was devolping and if your parked up for a while you can start the truck to recharge the tank if needed

I think maybe the reason the start tank fills last is because before the days of maxi brakes on trailers it can be very important to get the air up if get stuck on a steep hill

 

Paul 

Edited November 30, 2018 by mrsmackpaul

[thebaz]

I never needed a gauge as the air start was so reliable it never let me down more than a few times in 30years till now, so I fitted a gauge and a tap to the electric start solenoid so I had the best chance of dealing with the problem.

Now at least I can tell the status of the air tank at a glance.

 

Edited December 1, 2018 by thebaz

[thebaz]

The start tank seems to now hold air indefinitely as long as I deplete some of the air pressure from brake system before shutdown. I am going to replace the pressure protection valve and add another non return valve and see how that goes with the brake system at full pressure.

The brake system is losing pressure much faster than it use to, so now I am going to have to investigate that.

[JoeH]

My dad's 1980 DM uses one of those valves. Not sure if our R and RD trucks have them too, I assume they do.  It's on the firewall under the hood, near the steering column. It prevents the air from going to driver seat and other cab features until truck hits 60 psi. They're cheap, he has had some fail after a year or so I believe. Keep a couple on hand, I think he had one blow apart where it is "crimped" together. I believe it's probably an emergence feature to protect air brake system from auxiliary system leaks. Starting your truck after sitting for a month is a luxury, stopping on a downgrade with an air leak is a necessity.

[JoeH]

Just looked at it again, yours appears to be bolted together, ours are crimped together.