modern dual circuit air over hydraulic brakes

[patracy]

I thought combination valves were more for a disc/drum setup? We're (well I am) keeping drums all around. And I I think these MC's are 60/40. (I'll have to go look again) So we'd actually want the 60 to the rears and 40 to the front. Due to the extra volume. I could be completely wrong. But I know from some pics that @peashooter posted of his setup, you could see the MC plumbing directly into one of the air packs. At least, that's what it looked like?

 

[cbrTodd]

I thought combination valves were more for a disc/drum setup? We're (well I am) keeping drums all around. And I I think these MC's are 60/40. (I'll have to go look again) So we'd actually want the 60 to the rears and 40 to the front. Due to the extra volume. I could be completely wrong. But I know from some pics that @peashooter posted of his setup, you could see the MC plumbing directly into one of the air packs. At least, that's what it looked like?

I have dealt with some that were referred to as combination valves but were for disc brakes at all corners and really only had the function of the shuttle valve inside that shut off fluid flow to one circuit if one side had significantly more fluid go through it than the other. It just keeps more fluid in the reservoir in case of a failure downstream. I wonder if that was the case here?

 

[peashooter]

The “valve” that is shown between the MC and the Airpacs on the dual circuit trucks is actually just a shuttle style electric switch. Its called a differential valve switch. If the front or rear brake circuit fails and leaks then the pressure of that side drops off and the shuttle in the valve switch moves to close the switch (closes a circuit to the negative ground), that switch would be typically connected to a warning light on your dash so you know if there is a brake failure.
On the 87-89 dual circuit trucks and the A3 trucks this differential valve was a separate external part, simce that time some newer MC have this feature integrated. This switch is not needed to have working dual circuit brakes, its just there to alert you if a significant leak has just occured.
Attached is a drawing i made for a couple guys that show how some of these master cylinders with the built in differential switch can be hooked up.

 

[banditt1979]

I have dealt with some that were referred to as combination valves but were for disc brakes at all corners and really only had the function of the shuttle valve inside that shut off fluid flow to one circuit if one side had significantly more fluid go through it than the other. It just keeps more fluid in the reservoir in case of a failure downstream. I wonder if that was the case here?

I think so and I would like that type of fail safe on my truck.

Been busy with front knuckle repack and boot replacement so I will try to work on the combo valve part number search later this week.

 

[Jeepsinker]

Look at M105, M1061A1, and M750 trailers. They have a small master cylinder with an air pot for each axle, which is similarly sized and braked to an M35 axle. Use an M939 treadle valve and a standard off the shelf relay valve. Run one master/ brake pot on each axle and you have properly sized triple circuit brakes and still a single air circuit.
That's an idea of mine.

 

[patracy]

Look at M105, M1061A1, and M750 trailers. They have a small master cylinder with an air pot for each axle, which is similarly sized and braked to an M35 axle. Use an M939 treadle valve and a standard off the shelf relay valve. Run one master/ brake pot on each axle and you have properly sized triple circuit brakes and still a single air circuit.
That's an idea of mine.

I have two or three of those laying around. That was the path I was going to take at first. But I gave up on the idea. I don't like the thoughts of losing air pressure and having no physical connection to the brakes. Yes, I know in a air brake system you have no physical connection. But in the event of air failure the brakes apply. At least with the path I'm going with this MC, be it hydroboost or air packs, there is still a "physical" connection to the brakes. Unless there's a hyd. failure of course. I'm considering a pinion brake setup as well as a emergency brake option. Since the parking brake is a joke. It'd be more or less a "one time use" kinda thing. But I was thinking a rotor on the front axle and the rear most axle. Probably overkill to build that system in. But I'd like to have something akin to a modern vehicle where I have a e-brake that actually can do something.

 

[fleetmech]

I have two or three of those laying around. That was the path I was going to take at first. But I gave up on the idea. I don't like the thoughts of losing air pressure and having no physical connection to the brakes. Yes, I know in a air brake system you have no physical connection. But in the event of air failure the brakes apply. At least with the path I'm going with this MC, be it hydroboost or air packs, there is still a "physical" connection to the brakes. Unless there's a hyd. failure of course. I'm considering a pinion brake setup as well as a emergency brake option. Since the parking brake is a joke. It'd be more or less a "one time use" kinda thing. But I was thinking a rotor on the front axle and the rear most axle. Probably overkill to build that system in. But I'd like to have something akin to a modern vehicle where I have a e-brake that actually can do something.

I definitely get wanting to keep a direct connection, but for me, this concept was in a large part trying to add a dual circuit while getting away from the stock parts that can be hard to find, and cant be fixed out on the road.

If you want the extra piece of mind for the air system, the systems I initially described used a rear/ spring brake chamber to actuate the MC. In case of air loss, the spring would apply the brakes, just like in a big rig. Plumbing the spring release air through a standard yellow park brake valve would ensure that its entirely automatic and properly timed, or you could send air to the spring chamber via a snap switch like is used to actuate the front drive in the t-case. Or maybe use a restriction orifice in the spring air line(s) (depending on how you choose to plumb it) to slow the flow so any emergency release would be gradual.

If emergency planning is our biggest concern, you could use a setup like jeepsinker described, with a standard brake can on the front, and spring chambers on the center and rear axle masters. Plumbed up like a big rig with separate front and rear air circuits you would have the ultimate in redundancy: dual air circuits, triple hydraulic circuits, and spring emergencies on the rears.

 

[patracy]

I think I've decided the process I'm going to take. I was thinking about dual airpacks. That would make things "simple" when it comes to the trailer air side. But it also doubles the amount of air packs in the system which historically have been "problematic". Not to mention they're not cheap. I think I'm going to buy a hydroboost for the same Ford F700 these MCs are from. I'm going to offset mount the MC from the column due to space and the PS servo that is to come. But I think I'm going to use a pair of pillow blocks and a keyed shaft to hang the brake peddle from. That would allow me to easily make the pedal offset to the MC. And I should be able to also add in a treadle valve to actuate the trailer brakes.

 

[patracy]

On the drive home I just realized one trade off going that route. I know it's not often used, but the airpacks can have a air supply plumbed into them to make towbaring the deuce easier. (It's a add on to the line at the airpack, so it's not factory) The dual airpack route would allow for that. Ugh, I'm going to think myself into a corner on this.

 

[banditt1979]

On the drive home I just realized one trade off going that route. I know it's not often used, but the airpacks can have a air supply plumbed into them to make towbaring the deuce easier. (It's a add on to the line at the airpack, so it's not factory) The dual airpack route would allow for that. Ugh, I'm going to think myself into a corner on this.

It happens! You just need to choose the best route for you. I bought an extra rebuilt air pack a few months ago just in case the one I rebuilt didn't work, so I already had it which put me in the direction of the dual air packs. Finding that cheap mc sealed the deal for me. I also just purchased a pedal assembly for the single circuit with the bracket so I can modify that one and keep the original in place until the right time. If it doesn't work I may try your idea of a fire wall mounted mc.

 

[patracy]

I'm honestly thinking more to the hydroboost setup. There's been a many of deuces towbar'ed without air brakes on them. My only other truck that could "safely" do it would be the 5 ton. So it has a weight advantage. A single hydroboost on the wall and simply two lines down to tap into the rest of the system would be easier to plumb up.

 

[Dbluff]

As a thought, that's a little different.

Why not just find a dual port master cylinder that can be put into the stock location of the current and set up the brake lines to the front and rear? Seems it would work.

 

[banditt1979]

As a thought, that's a little different.

Why not just find a dual port master cylinder that can be put into the stock location of the current and set up the brake lines to the front and rear? Seems it would work.

Thats what I am trying to do. The problem is the dual port mc that is needed is too long to fit in the original location, so a custom bracket is needed. The air force and A3 deuces have a bracket that can work but those are very difficult to find. I'm going to try modifying the bracket to make the mc fit.

You also need the extra air pack, extra air tank, and possibly the combination valve to make sure fluid is stopped when a line gives out.

 

[patracy]

As a thought, that's a little different.

Why not just find a dual port master cylinder that can be put into the stock location of the current and set up the brake lines to the front and rear? Seems it would work.

There isn't one. Due to the space allowed in the area. And the bracket design. Then you still have to deal with a second airpack.

 

[patracy]

Can someone give me some advice on the Hydro-max booster setup? I am leaning that way because with no hydraulic pressure the motor will kick in to assist. My wrinkle is the hyd. plumbing circuit. From what I see, the outlet of all these hydro-max boosters are for low pressure returns. I plan to use a Sweet Mfg steering servo to add power steering to my truck. From what I understand, the Hydroboost setups get the first go at hyd pressure from your pump, then pass that to the power steering system. But with the hydro-max setups simply having a nipple outlet for return, you can't expect the pressure demands of the PS to not blow that line off/up. I found this diagram with the hydro-max setup. But it's plumbed reverse to the common setup.



Would that truly be acceptable in this system? I would be concerned about braking and steering at the same time and there not being as much pressure available for the brakes. But I suppose that's the reason for the hydromax pump, it could kick in if there's low pressure?

 

[patracy]

I guess I'm overthinking things again. Because I found other examples of the same routing.

 

[Dbluff]

Guy on Youtube put a freigthliner hydrobooster on his M35a2, or at least I think.

So to the above, you can't use a single air pack with a dual port MC? Seems if a single port like ours works, why would a dual port MC need another air pack?

 

[fleetmech]

Guy on Youtube put a freigthliner hydrobooster on his M35a2, or at least I think.

So the above, you can't use a single air pack with a dual port MC? Seems if a single port like ours works, why would a dual port MC need another air pack?

The guy in that vid, IIRC, is running the electric backup pump as his sole booster.

The airpack system on these trucks doesn't work like a standard modern power brake system. The master cylinder is directly operated by the pedal lever which sends fluid down to the separate airpack unit. This fluid movement operates a control valve which engages the airpack. The 'pack is a big air cylinder, pushed by the trucks compressed air supply, which in turn pushes on a second, integral, master which then pushes high pressure fluid out to the wheels. Its one line in and one out, with no provision for splitting.

Its an archaic system, i think it was developed by Midland even before the deuce, and was not often used on road going machines. It was generally used on construction equipment, like scrapers and wheel loaders, and it was phased out a long time ago.

*UPDATE* I just found out that Beaver/ Monaco built bus motorhomes into the 90's using multiple single pot masters run by air cans. Exactly the system we're thinking of!

Haldex Series III (p/n N37216C) referred to as the Haldex Pressure Converter.

 

[banditt1979]

The guy in that vid, IIRC, is running the electric backup pump as his sole booster.

The airpack system on these trucks doesn't work like a standard modern power brake system. The master cylinder is directly operated by the pedal lever which sends fluid down to the separate airpack unit. This fluid movement operates a control valve which engages the airpack. The 'pack is a big air cylinder, pushed by the trucks compressed air supply, which in turn pushes on a second, integral, master which then pushes high pressure fluid out to the wheels. Its one line in and one out, with no provision for splitting.

Its an archaic system, i think it was developed by Midland even before the deuce, and was not often used on road going machines. It was generally used on construction equipment, like scrapers and wheel loaders, and it was phased out a long time ago.

*UPDATE* I just found out that Beaver/ Monaco built bus motorhomes into the 90's using multiple single pot masters run by air cans. Exactly the system we're thinking of!

Haldex Series III (p/n N37216C) referred to as the Haldex Pressure Converter.

Thats good and interesting info, thank you

 

[patracy]

So to the above, you can't use a single air pack with a dual port MC? Seems if a single port like ours works, why would a dual port MC need another air pack?

No. The airpack has only one inlet and one outlet for the hydraulic (brake) fluid. The Air Force dual circuits use two airpacks.