Servo Brake Conversion

[Bacon4x]

The M35 brakes are a nonservo style brake or a leading-trailing brake. The brake shoes on a leading –trailing brake are mounted independently, each shoe with its own anchor point at the bottom and the wheel cylinder at the top.
A servo style brake uses one anchor point for both shoes and is arranged so that the shoes apply pressure to each other during braking action. In other words, when the brakes are applied the primary shoe contacts the spinning drum and attempts to rotate with the drum, as the primary shoe rotates with the drum it forces the secondary shoe to apply pressure to the drum.
A servo style brake requires much less input force to produce the same “grab” as a leading-trailing brake and is the style of drum brake you find on most modern trailers and vehicles.
So, that being said I am wondering what it would take to convert the M35 brakes to servo style brakes.
At first this sounds like it would be a total cluster, but perhaps it would not really be that bad if a guy could find a common vehicle with 15”x3” servo style drum brakes. An older F800 or C60 etc… for which parts are cheap and easy to get? I don’t know about totally swapping drums and backing plates, but perhaps it would not take all that much modification to adapt the civilian truck brake hardware onto the M35 backing plate. You would need to move the location of the wheel cylinder, add the single anchor point, and drill a few holes for some retaining springs. Other than that since we don’t need a parking brake inside the drum there is not much to it.
Maybe easier said than done, thoughts?

 

[JasonS]

My 1947 chevy pickup had the huck non-servo brakes and my 1948 was converted to the bendix servo brakes and I really couldn't tell a difference. Might be a lot of work to find out the same on an M35.

 

[Bacon4x]

JasonS was there also not much difference in terms of the force required at the pedal? I don't doubt your observation, I'm just trying to consider all angles.

On my truck I measured the pedal input force versus brake line pressure using a scale on the pedal and pressure gauge on a brake line. I measured the force at the pedal to be about 150 lbs to create 1000 psi line pressure, 170 lbs to create 1500psi line pressure, and about 185 to 190 lbs to create 1800 psi line pressure. When I pushed really hard on the pedal I could get 2000 psi line pressure, but could not hold steady enough to read the scale.

If you were to install a hydroboost or hydromax system you will more easily achieve the 2000psi line pressure or higher, but you will still be working with the same size "lever" at the wheel. Also, some folks might question the safety of increasing line pressure that high in a system designed to run around 1800psi.

In theory a servo brake design will increase your "lever" at the wheel, but still allow you to operate with reasonable brake line pressures. However, your observation might suggest that the improvent is not significant enough to make it worth while.

Can anyone else comment on this?

 

[m-35tom]

if all you want to do is decrease the required pedal pressure, just reduce the diameter of the air piston on the side of the airpack. it is a fixed pressure regulator, and the balance values can be changed easily by pressing in a sleeve and installing a smaller piston. results in much lower pedal effort.

tom

 

[JasonS]

JasonS was there also not much difference in terms of the force required at the pedal? I don't doubt your observation, I'm just trying to consider all angles.

On my truck I measured the pedal input force versus brake line pressure using a scale on the pedal and pressure gauge on a brake line. I measured the force at the pedal to be about 150 lbs to create 1000 psi line pressure, 170 lbs to create 1500psi line pressure, and about 185 to 190 lbs to create 1800 psi line pressure. When I pushed really hard on the pedal I could get 2000 psi line pressure, but could not hold steady enough to read the scale.

If you were to install a hydroboost or hydromax system you will more easily achieve the 2000psi line pressure or higher, but you will still be working with the same size "lever" at the wheel. Also, some folks might question the safety of increasing line pressure that high in a system designed to run around 1800psi.

In theory a servo brake design will increase your "lever" at the wheel, but still allow you to operate with reasonable brake line pressures. However, your observation might suggest that the improvent is not significant enough to make it worth while.

Can anyone else comment on this?

I know that the servo is SUPPOSED to reduce pedal effort, it just wasn't obvious to me.

 

[JasonS]

if all you want to do is decrease the required pedal pressure, just reduce the diameter of the air piston on the side of the airpack. it is a fixed pressure regulator, and the balance values can be changed easily by pressing in a sleeve and installing a smaller piston. results in much lower pedal effort.

tom

Somone on SS hinted at a much simpler method of reducing pedal pressure but I don't remember the details.

 

[doghead]

Do you have trouble pressing your brake pedal effectively now?

Are your brakes recently serviced and functioning like they should be?

I've never considered the pedal requiring more than I have, or less that satisfactory(with a well maintained brake system).

 

[Bacon4x]

Everything on my brake system is either new or rebuilt with the exception of the hard lines, hardware, and the shoes. The shoes are by no means brand new, but they are also not worn out or oil soaked.

I have not drove that many of these trucks for comparison, but I would not say my brakes perform all that terrible. When its empty I can skid the tires, but if its loaded and I needed a panic stop I don't think it would be able to skid the tires. Maybe my brakes are not performing at 100% their capability. If the entire brake system were band new should the truck be able to skid the tires on pavement when loaded (loaded within its rating), and have you actually done it or seen it done?

I don't have any issues pushing my brake pedal nor does it bother me that it takes a bit more effort than a modern truck for example. A reduced pedal effort is not my only desire. I was trying to think of an improvment that would not push the limits of brake line pressure in order to achieve better stopping power.

The way I see it the only ways to increase stopping power would be through increasing the force acting on the brake shoes, increasing the drum size, or increasing the coefficient of friction of the brake shoe lining.

Increasing drum diameter is not going to be practical. Changing the brake shoe lining to a higher friction compound could be something to look into. Increasing the force acting on the shoes can be achieved through increased line pressure, a larger diameter wheel cylinder, or perhaps through a servo design.

 

[m-35tom]

i guess i don't understand then why you are asking about servo style brake shoe mounting. the first thing anyone should consider is going to split brake system as the single is just not safe. if you then want more stopping power, just increase the pressure.

 

[m16ty]

The deuce brake system does have it's faults (single circuit) but IMO pedal pressure isn't one of them. Sounds like a whole lot of work to solve a problem that doesn't exist.

My deuce will stop as well as any modern big truck. As with any larger truck, you've got to allow plenty of stopping room (especially when loaded).

 

[RAYZER]

The possibility that parts would be cheaper and readaly available is intriguing. Doe's a servo system use a double, or single piston wheel cylinder? Could the stock backing plate be converted?

 

[Bacon4x]

m-35tom I would totally agree that the single system is not safe and plan to split my system, but aside from that issue, I think the safety of increasing line pressure is debatable. If I am not mistaken I believe a hydroboost system will achieve around 2000psi line pressure. According to my pressure measurements on the current system I too can achieve 2000psi if I really push hard on the pedal, but even at that pressure the wheels will not lock up when loaded. (Is that specific to my truck or typical for everyone?)

If we were to use a hydromax system then we could achieve much higher line pressures than 2000psi, but some people would question the safety of running that high of pressure in a drum brake system. Therefore, I was wondering if a servo style brake shoe mounting would increase performance without pushing the limits of line pressure. You could also increase the wheel cylinder diameter but would that gain outperform the servo configuration or not?

 

[Bacon4x]

m16ty when loaded can your deuce lock up the wheels? My understanding, and maybe its not true, is that most modern trucks, with the excption of antilock systems etc.., have powerful enough brakes to lock the wheels when fully loaded.

Also, Im not suggesting we need to go skidding around everywhere, but its the times when you don't plan for lots of stopping room that it would be nice to have the extra stopping power.

Rayzer, I think the servo configuration could be setup with either type piston as long as the shoes float within the drum which allows them to apply pressure to one another. I would hope the stock backing plate could be converted but I don't know. Not sure anyone would want to get into changing out all backing plates, although maybe that would be easier than modifying them.

 

[jesusgatos]

There is some good tech related to line-pressure and brake system components in the HydroMax thread.

 

[m16ty]

m16ty when loaded can your deuce lock up the wheels? My understanding, and maybe its not true, is that most modern trucks, with the excption of antilock systems etc.., have powerful enough brakes to lock the wheels when fully loaded.

Honestly, I haven't tried to lock the brakes loaded. I think it might if you stay under the 5-ton max payload rating. You can overload a deuce fast if you start hauling stuff like gravel and such in the bed.

 

[rustystud]

Hi Bacon4x I'm coming to this party late, maybe your not looking anymore, but I believe the modern system is better. You must be a mechanic also . I've worked in the over the road truck repair business for years and all modern trucks when the brakes are applied with force will lock-up the wheels, seen any jack-knifed tractor riggs ! That's why they teach you to jab your brakes in a panic stop. Yes I also have a class
A cdl with triples endorsement. I also agree with you about the line pressure exerted when using that much force to stop. I've looked into using a "G" grade brake lining or maybe something softer. The only problem
is that you go through linings much faster. You can go to any truck brake shop and buy linings and rivets and do the job yourself or have them do it for you. As far as the servo style goes, you will need longer brake shoes and then the adjustable anchor between them. I think you would also have to change out the brake cylinder as it is longer then the normal size for the diameter of drum. Maybe you can just find
a complete system from another truck like an IHC or Ford. I lucked out and my truck came with a dual brake system and so far stopping is not a problem. I haven't loaded it down yet though

 

[JasonS]

My 1947 chevy pickup had the huck non-servo brakes and my 1948 was converted to the bendix servo brakes and I really couldn't tell a difference. Might be a lot of work to find out the same on an M35.

It's been a year since I posted this. I spent some time on the 1947 brakes over the summer. Want to know what REALLY improved the brakes? Proper adjustment. Instead of feeling for drag which gave poor results, I looked through the adjust holes and adjusted for equal shoe movement (non servo). Now each brake shoe is working optimally and it stops like it has power brakes. I really need to do this on my M35....

 

[ducer]

The big advantage of the self energizing brake system is the self adjusters which is why they were put into cars in the first place.
Instead of having to take the car in to have the brakes adjusted when the pedal was starting to get low, the automatic adjusters did it automatically everytime you backed up when the brake lining to drum clearance was to big.

Denny