m35 brakes

[pigpen60]

If you lose air pressure on a m35 do you lose the brakes? Limited air over hydraulic experience.

 

[steelandcanvas]

No, they are just a lot harder to actuate.

 

[swbradley1]

That is an understatement.

 

[pigpen60]

Thank you for reply.

 

[Valence]

That is an understatement.

This is a very, very true statement.

How much do you weigh, because even standing on them is not enough.

I've been in friend's deuce going down a steep grade about 10-20 mph, with less than 30PSI of air. He was standing on the brakes, pulling up on the steering wheel for more force. I'm surprised the steering wheel didn't break for how much it was bending. The only thing that stopped us was sacrificing the clutch and associated components.

 

[rustystud]

This is a very, very true statement.

How much do you weigh, because even standing on them is not enough.

I've been in friend's deuce going down a steep grade about 10-20 mph, with less than 30PSI of air. He was standing on the brakes, pulling up on the steering wheel for more force. I'm surprised the steering wheel didn't break for how much it was bending. The only thing that stopped us was sacrificing the clutch and associated components.

I have a brake pressure dash gauge on my deuce and without the engine running (no air) I can get it up to 800psi. With the engine running (full air) I can get it up to 1600psi .

 

[swbradley1]

I have a brake pressure dash gauge on my deuce and without the engine running (no air) I can get it up to 800psi. With the engine running (full air) I can get it up to 1600psi .

I don't think the difference in pressure is a one-for-one comparison, I believe it is more exponential IMHO. I could be wrong but with air it doesn't take that much pressure on the brake pedal to stop. No air and good freaking luck and as big as I am it is darn near impossible to stop.

 

[gimpyrobb]

I've been in friend's deuce going down a steep grade about 10-20 mph, with less than 30PSI of air. He was standing on the brakes, pulling up on the steering wheel for more force. I'm surprised the steering wheel didn't break for how much it was bending. The only thing that stopped us was sacrificing the clutch and associated components.

Kinda got what he deserves if you ask me. How long does it take to build full pressure? Maybe 3mins?

 

[clinto]

Small issue:

"Air over hydraulic" is like what an M105 has. Loss of air = total loss of brakes

"Air assisted hydraulic" is what the M44 series has. Loss of air = loss of power brakes.

That being said, when I got my first deuce, I drove it down my driveway with no air and tried to stop it so I'd know what to expect in an emergency. Let's just say I'm now really careful with my air system and give it proper maintenance. Like the posters above said, with both of my hands on the wheel and pushing with all my force on the pedal, I could barely get it stopped. The airpack is big deal, provides a lot of boost.

 

[DavidWymore]

How much does the park/emerg brake help?

 

[clinto]

How much does the park/emerg brake help?

When the vehicle is underway?

Nearly none. I'd say if you're going more than maybe 5 mph, you will apply it, you will feel a mild lurch, a bad smell will occur and you will continue hurtling towards whatever obstacle you're going to hit.

 

[gimpyrobb]

I concur.

 

[71DeuceAK]

Interesting, the emergency-brake thing is something this newbie here has wondered for the last year himself.

I did once have the (slightly hair-raising?) experience of riding along in a friend's M49 tanker Deuce, whose brakes didn't work. It was, however, not on public roads. He was an experienced driver though...an experienced Deuce driver, too. He was quizzing me, who am just learning to drive..."What do I do if I have to stop anyway?. I guessed "shift down as far as you can and into neutral?". I think it was actually "sacraficing the clutch"...am I right? He also told me on the same drive (In fact THE first thing he told me): "When you get one of these (He knows I plan on owning one) go through the brake system immediately! They have these real chintzy lines that tend to blow...they're steel, maybe stainless will work, I don't know". I commented "I wonder about copper?"

The one thing I DONT' like about Deuces IS their brake system!

 

[98G]

A park brake is not designed nor intended as an emergency brake.

Emergency brakes are the springbrakes in a true airbrake system.

 

[71DeuceAK]

The ones where you involuntarily stop, right?

The one thing with those I've wondered is: What about the involuntary stop at an inopportune time? (freeway in rush hour, etc?)

 

[98G]

The ones where you involuntarily stop, right?

The one thing with those I've wondered is: What about the involuntary stop at an inopportune time? (freeway in rush hour, etc?)

Yup, that's an issue. An involuntary stop is better than none...

But here's how it actually plays out most of the time: 1) fssssssssss!!! and your air gauge is dropping rapidly. You take a couple of seconds to pick your stopping place and put the truck there, and apply the brakes as normal. The springbrakes also apply and there you sit until resolved.

The thought process behind it is that a truck that won't move is safer than a truck that won't stop.

 

[71DeuceAK]

Right...I can see why. Chances are you won't be rear-ended by a large truck in that case anyway, more likely a small car.

 

[rustystud]

I don't think the difference in pressure is a one-for-one comparison, I believe it is more exponential IMHO. I could be wrong but with air it doesn't take that much pressure on the brake pedal to stop. No air and good freaking luck and as big as I am it is darn near impossible to stop.

What I'm saying is I have a brake pressure gauge on my dash. When I apply the brakes without air pressure I can only get it to go to 800psi standing on it. With the truck full of air pressure (120psi) I can get 1600psi pressure from that gauge easily. Does that mean that there is a 2 to 1 ratio ? I don't know. I can only say what my gauge is saying. I do know that it is not exponential since the piston doesn't change shape. It is and will always be the same size. Just like a hydraulic jack.

 

[SP5]

Deuce brakes are "intresting", being basically a 1930s design. Because they use a single wheel cylinder and duel fixed anchor pins there is only one leading shoe, and one trailing shoe for each drum, and the shoes switch depending on vehicle direction when the brakes are applied.
When you are going forward and apply the brakes, the forward shoe becomes the leading shoe, when you are in reverse, the rear shoe becomes the leading shoe, this also means that the brakes work equally well, (or bad), in either direction.
But in reality, you don't often have to make hard/fast stops from high speed in reverse, and so the Deuce design, while strong and simple is lacking in stopping power by modern standards.
In most any type of drum brake, the leading shoe does most of the work. With the fixed anchor pin design, you effectivly give-up ~25>30% of your braking power,, but with enough power assist rhe system works.
The modern system, (Kelsey-Hayes/Bendix)whereby the shoes are connected together by a "floating" anchor allows both of the shoes to "cam" themselves into the drum, resulting in a large increase in stopping power for the same brake-line hydraulic pressure.
The "floating anchor" system was originally designed for aircraft, which, (as planes got bigger/heavier/faster landing speeds), needed brakes that would "stop" without power assist.

 

[JasonS]

What I'm saying is I have a brake pressure gauge on my dash. When I apply the brakes without air pressure I can only get it to go to 800psi standing on it. With the truck full of air pressure (120psi) I can get 1600psi pressure from that gauge easily. Does that mean that there is a 2 to 1 ratio ? I don't know. I can only say what my gauge is saying. I do know that it is not exponential since the piston doesn't change shape. It is and will always be the same size. Just like a hydraulic jack.

TM 9-1819B (page 192) states that there is a factor of 17x between the applied air pressure and the resulting hydraulic pressure.