69 R Model

[69R700]

Hello,

I have a 69 R Model Mack. 2-3 weeks ago I took the truck for a ride, hit a couple of nice sized pot holes and by the time I got back to the yard I was losing air pressure (getting up to about 50 PSI of air). If I rev the engine, I can get up to a little over 60 PSI. I found air was coming out of the bottom of the foot valve while the truck was shut off. Today I put in a brand new foot valve and still losing air out of the bottom of the foot valve. Somebody told me to look at the valve behind the cab ... it has 3 copper lines feeding into the bottom and the 2 trailer air lines come out of the top. Can anyone recommend anything else I can look for?

Thanks.

- Bob

[theakerstwo]

Hello,

I have a 69 R Model Mack. 2-3 weeks ago I took the truck for a ride, hit a couple of nice sized pot holes and by the time I got back to the yard I was losing air pressure (getting up to about 50 PSI of air). If I rev the engine, I can get up to a little over 60 PSI. I found air was coming out of the bottom of the foot valve while the truck was shut off. Today I put in a brand new foot valve and still losing air out of the bottom of the foot valve. Somebody told me to look at the valve behind the cab ... it has 3 copper lines feeding into the bottom and the 2 trailer air lines come out of the top. Can anyone recommend anything else I can look for?

Thanks.

- Bob

I am thinking that you did not get any lines mixed up when you replace the foot valve so the only other thing i can think of and i am only thinking but if this is leaking when your park brakes is released then you may have aparking brake chamber leaking inside but it should exhaust off fromk the relay. The other thing i can think of is if your trailer hand vavle is pulled and then your sliding check valve would be bad or some call it 3 way check valve. Make sure your trailer hoses is not connected to each other at the ends. If the valve on the back of the cab or tractor protection valve was bad then if your red knob was pushed in and the valve was bad it could leak back.

[ThaddeusW]

First off is the plumbing all original? If original it should be a single circuit air system and the foot valve should have only 2 air lines hooked to it.

The only way a good foot valve will leak from the exhaust port is when your getting air fed back to the valve from somewhere else. You have to trace the line from the foot valve delivery port (the port that feeds the brake chambers) and see where it leads to. You should/might have a relay valve at the rear to feed each of the four brake chambers on the drives. That could be the problem.

I would first disconnect the delivery port from the foot valve and run the engine to see if any air is coming out of the delivery hose. There shouldn't be as long as you foot is off the valve and no air should come from the foot valve either. So if that happens trace the line and see what it hooks to next.

Do the brakes feel like they are dragging? I ask because if there is air pressure in the brake lines then they might be slightly applied. Do you have an air dryer or filter from the compressor?

[69R700]

Hi, Thad -

Yes, the brakes do feel like they're dragging a little bit. I don't have an air dryer but do have a filter off the compressor. It's a Detroit diesel 8v71 so it's a little different from the Mack system. I'm planning on selling the truck and now this problem popped up ... I'm really not familiar with this so I haven't been able to repair it. Any information is appreciated. In the meantime, I'll just keep plugging along.

Thanks.

- Bob

P.S. Thad ... I think you're from Queens??? I'm on Long Island. Do you do any repair work on the side?

First off is the plumbing all original? If original it should be a single circuit air system and the foot valve should have only 2 air lines hooked to it.

The only way a good foot valve will leak from the exhaust port is when your getting air fed back to the valve from somewhere else. You have to trace the line from the foot valve delivery port (the port that feeds the brake chambers) and see where it leads to. You should/might have a relay valve at the rear to feed each of the four brake chambers on the drives. That could be the problem.

I would first disconnect the delivery port from the foot valve and run the engine to see if any air is coming out of the delivery hose. There shouldn't be as long as you foot is off the valve and no air should come from the foot valve either. So if that happens trace the line and see what it hooks to next.

Do the brakes feel like they are dragging? I ask because if there is air pressure in the brake lines then they might be slightly applied. Do you have an air dryer or filter from the compressor?

[HK Trucking]

I agree with Glenn.

Most times it's a problem with the double check valve that causes this.

[Kav]

I agree with Glenn.

Most times it's a problem with the double check valve that causes this.

Hi everybody, When I was working My 1969 R Model I had a similar problem. It turned out to be the seals between the Service and Maxi brake chambers were leaking and the air was coming back through the vent on the treadle valve. That model had a very simple arrangement where the Parking brakes discharged through the service system so that when you put your foot on the service brakes, it released the pressure from the spring brakes. I don't know whether this was only on the Australian models, but it was much simpler than having the double check valve. I change the Maxi -brake units and stopped the leak. I too had already changed the foot valve but it made no difference. Best regards Michael.

[Kav]

I agree with Glenn.

Most times it's a problem with the double check valve that causes this.

Hi everybody, When I was working My 1969 R Model I had a similar problem. It turned out to be the seals between the Service and Maxi brake chambers were leaking and the air was coming back through the vent on the treadle valve. That model had a very simple arrangement where the Parking brakes discharged through the service system so that when you put your foot on the service brakes, it released the pressure from the spring brakes. I don't know whether this was only on the Australian models, but it was much simpler than having the double check valve. I change the Maxi -brake units and stopped the leak. I too had already changed the foot valve but it made no difference. Best regards Michael.

[ThaddeusW]

Hi, Thad -

Yes, the brakes do feel like they're dragging a little bit. I don't have an air dryer but do have a filter off the compressor. It's a Detroit diesel 8v71 so it's a little different from the Mack system. I'm planning on selling the truck and now this problem popped up ... I'm really not familiar with this so I haven't been able to repair it. Any information is appreciated. In the meantime, I'll just keep plugging along.

Thanks.

- Bob

P.S. Thad ... I think you're from Queens??? I'm on Long Island. Do you do any repair work on the side?

Yup Queens. I do some mechanic work on the side, usually light stuff for friends. Mostly very very busy so its tough for me to come out and help. I was helping out another guy from Long Island but I only got out to him once. I have no problem helping a fellow Mack enthusiast when I can but time and work are a problem.

Herb, that sounds plausible but he has the very same problem as he did before he replaced the valve so there is air leaking into the lines from somewhere else. Michael has a very good point about air leaking between the service and spring chambers. Never thought of that one. Without working knowledge of the air system it might be a pain to find the leak. Trace that line Bob and note what it feeds. From there we can help.

And you still need to tell us if you have spring type parking brakes that are activated with a dash valve.

[69R700]

Yup Queens. I do some mechanic work on the side, usually light stuff for friends. Mostly very very busy so its tough for me to come out and help. I was helping out another guy from Long Island but I only got out to him once. I have no problem helping a fellow Mack enthusiast when I can but time and work are a problem.

Herb, that sounds plausible but he has the very same problem as he did before he replaced the valve so there is air leaking into the lines from somewhere else. Michael has a very good point about air leaking between the service and spring chambers. Never thought of that one. Without working knowledge of the air system it might be a pain to find the leak. Trace that line Bob and note what it feeds. From there we can help.

And you still need to tell us if you have spring type parking brakes that are activated with a dash valve.

Hi Thad yes I have spring type brakes ,the truck has a red dash valve for air reserve which is in the pulled out position,in between the seats is the wet and dry pavement selector ,the tractor brakes,and the tractor protection valve normal/emergency which is in the normal position.Thad if I release the parking brake and throw the tractor protection valve to emergency shouldnt i get air coming out the red air line to the trailer?

[69R700]

Hi Thad yes I have spring type brakes ,the truck has a red dash valve for air reserve which is in the pulled out position,in between the seats is the wet and dry pavement selector ,the tractor brakes,and the tractor protection valve normal/emergency which is in the normal position.Thad if I release the parking brake and throw the tractor protection valve to emergency shouldnt i get air coming out the red air line to the trailer? Spring brakes only on the rear axle

[ThaddeusW]

Hi Thad yes I have spring type brakes ,the truck has a red dash valve for air reserve which is in the pulled out position,in between the seats is the wet and dry pavement selector ,the tractor brakes,and the tractor protection valve normal/emergency which is in the normal position.Thad if I release the parking brake and throw the tractor protection valve to emergency shouldn't i get air coming out the red air line to the trailer?

I believe when in the emergency position you should have air coming out the emergency/supply line (nowadays the red line). You have the old system which unfortunately I have no diagrams for. I will dig around for some information and get back to you. In the mean time if anyone has anything to add please do .

[69R700]

I believe when in the emergency position you should have air coming out the emergency/supply line (nowadays the red line). You have the old system which unfortunately I have no diagrams for. I will dig around for some information and get back to you. In the mean time if anyone has anything to add please do .

Hi Thad capped off the rear brake chambers last night and still the same problem but I think I found the problem the valve behind the cab on the drivers side mounted in between the frame rails to cross member has 4 air lines going to it and every time I step on the brake air is pouring out the top of valve near the fittings all cooper. I going to replace the valve this week and take some pictures of some of the other valves on the truck which I think one of them is a moisture separator. Tractor also has front brakes . Hope some of these valves are still availble ,there all orignal.

Thanks

Bob

[ThaddeusW]

Hi Thad capped off the rear brake chambers last night and still the same problem but I think I found the problem the valve behind the cab on the drivers side mounted in between the frame rails to cross member has 4 air lines going to it and every time I step on the brake air is pouring out the top of valve near the fittings all cooper. I going to replace the valve this week and take some pictures of some of the other valves on the truck which I think one of them is a moisture separator. Tractor also has front brakes . Hope some of these valves are still availble ,there all orignal.

Thanks

Bob

Good stuff! I was wrong about the tractor protection valve. After some research I found out that when the tractor protection valve is in the normal position you should have air spitting out of the emergency/supply line. That will release the trailer brakes. It can be plumbed to work with the tractor brakes so the tractor brakes park the whole rig or it can be plumbed by itself to operate regardless if the tractor brakes are set of not.

[theakerstwo]

Dont forget that the new valve can have some dirt or rust or telfon tape under the spool in the new valve. So many times rust from a air line that is in bad shape or rust from the air tank can get in the old valve make it leak as well as the new valve. glenn

[69R700]

Good stuff! I was wrong about the tractor protection valve. After some research I found out that when the tractor protection valve is in the normal position you should have air spitting out of the emergency/supply line. That will release the trailer brakes. It can be plumbed to work with the tractor brakes so the tractor brakes park the whole rig or it can be plumbed by itself to operate regardless if the tractor brakes are set of not.

Hi Thad when I build up air pressure and throw the tractor protection valve from normal to emergency air pours out of the red trailer line when the tractor brakes are in the released position.

Bob

[ThaddeusW]

Hi Thad when I build up air pressure and throw the tractor protection valve from normal to emergency air pours out of the red trailer line when the tractor brakes are in the released position.

Bob

Someone might have mounted the valve upside down. Normal means supply air, emergency means release the air which would apply the trailers spring brakes.

[theakerstwo]

Hi Thad when I build up air pressure and throw the tractor protection valve from normal to emergency air pours out of the red trailer line when the tractor brakes are in the released position.

Bob

Thad is right the valve is working backward. Emergency means you want to stop the trailer so it cuts the air and sets the trailer brakes. Try takeing the plate off and turning it up side down but then you are going to stand on your head to read the label.

[69R700]

Thad is right the valve is working backward. Emergency means you want to stop the trailer so it cuts the air and sets the trailer brakes. Try takeing the plate off and turning it up side down but then you are going to stand on your head to read the label.

Hi replaced the relay valve last night and still have air coming out of the foot valve ,I also took apart the moisture separator Wagner It has 2 lines connected to it one comes off a t and the other connects right to it ,then there is elbow plugged so how does it expell moisture ? inside valve there was no spring under that nut on top just a diaphram .

Iam going to check the trolley valve, mechanic said a lot of people overlook it I HOPE .

Bob

[theakerstwo]

Hi replaced the relay valve last night and still have air coming out of the foot valve ,I also took apart the moisture separator Wagner It has 2 lines connected to it one comes off a t and the other connects right to it ,then there is elbow plugged so how does it expell moisture ? inside valve there was no spring under that nut on top just a diaphram .

Iam going to check the trolley valve, mechanic said a lot of people overlook it I HOPE .

Bob

Try taking out the snap/ring in the bottom of the new foot valve and remove the spool and see if you dont have some rust or tefon tape under the rubber seal and see if that dont repair. If there is only one supply and one deliver to that foot valve and it has air coming from the exhaust it means your spool is leaking in the foot valve. glenn

[69R700]

Try taking out the snap/ring in the bottom of the new foot valve and remove the spool and see if you dont have some rust or tefon tape under the rubber seal and see if that dont repair. If there is only one supply and one deliver to that foot valve and it has air coming from the exhaust it means your spool is leaking in the foot valve. glenn

Thanks everyone for there help. The Wagner moisture ejector was the culprit tryed to get another one but they are no longer availble so had to bypass it and that solved the problem.

Bob

[ThaddeusW]

Thanks everyone for there help. The Wagner moisture ejector was the culprit tried to get another one but they are no longer available so had to bypass it and that solved the problem.

Bob

Glad you fixed the problem Bob.