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.

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.

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.

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.

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?

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.

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.

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.

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.

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.

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.

I think 2-3 weeks. I could start the truck after months without a problem for it's whole life until recently

Went to start the truck today, out of air in the start tank. Grrrrrrr!!! Will have to investigate further now.

I am not losing any air now from the start system, only the brake/air supply side of things. I didn't need the truck for a few weeks and it started up easily, but the brake side is down to 60psi after 24hrs.

I will check that out Paul. The truck dropped from 125PSi to 100psi in around 8 hours which is not bad but it used to be better than that. It is nice if I can hit the starter and idle out of the driveway within minutes to keep the neighbors happy if possible.