404.115 Air over Hydraulic Brakes

ACAD_Cowboy · 11-04-2009, 05:07 PM

With the air over hydraulics braking system, assuming a fully bled down air system, what should I expect to feel in terms of pressure at the pedal?

Long story short I'm going to be spending a lot time with 404 I shared with you guys, doing some freelance work for it's owner. So I had some time to kill and wanted to try to develop a better idea about the truck, perhaps get the hood open and check the systems.

So I got in the cab and checked the various pedals and levers. Throttle is pretty NFU due to carb gumming but the pedal travel is okay, same with the hand throttle.

Clutch feels good, no drag or excessive force (compared to my diesel VW it's a monster but compared to my CJ it's about normal) and that leaves the braking system. The transmission is floating somewhere between this way and that way and most likely between gears and until I can spin the input I'm not going to try to force a gear. I suspect that to move it around someone discovered that if you put the direction selector in N and the gearbox in N it rolls around on the wonder tires (that never ever leak ever) without making funny noises.

The brake pedal however seems to have very limited resistance. I didn't feel much of anything so I did not try to pump the system up. The E-brake has some feedback but I suspect the shoes are crusted in place.

On the pedal, does the system require the pneumatics to be on line to function worth a damn? Do I just assume the system to be a write off and prepare to replace and rebuilt from the pedal back?

Public_AenimA · 11-05-2009, 01:12 AM

It should function like any other assist system. With no air, it should be rock hard. I would plan on replacing or rebuilding all of the hydraulic parts of the brake system.

ACAD_Cowboy · 11-05-2009, 08:05 AM

Well you win some and lose some I guess.

I find myself wishing this guy took better care of it versus just letting it biodegrade.

Is it worth it trying to pump the system up or am I risking massivly screwing the master cylinder?

Public_AenimA · 11-05-2009, 11:09 AM

All the hydraulic leaks I found in my system were from pitting on the bottom of the cylinder. Just little rust spots. It is already broke... how much worse can you make it? If the bore is pitted it is probably time to replace the master internals anyway. The master rebuild I bought from Eurotech was about 200$ and replaced everything but the casting. Then I had to hone the piss out of my cylinder but it worked when I was done. I few hours honing seemed better that 600$ for the new complete Master.

ACAD_Cowboy · 11-05-2009, 02:25 PM

Well as I mentioned, the hood is caught up so for all I know it has a down stream leak and is dry. Until I can really crawl all over it it's all just guess work.

On the related note, I penned a deal today with it's owner to do some freelance work on an extended basis so I'll have a lot of time with the truck to very carefully inspect it and not have the questions of just WTF am I doing. Now it'll be more like oh hey, how ya' doing.

Getting the hood up and looking at fluid systems are my short term goal.

And who knew you could barter for a unimog?!

bigmellon · 11-05-2009, 09:46 PM

You can make a fitting to fill the air tank from a shop compresor. If you do this and air gushes from the master cylinder... You need to get it rebuilt. Be careful, I rebuil done and the new parts didn't fit the best...

Do some searching on brake issues here and on the benz forum and you can find a lot of data. My 404.0 was a royal pain and there is a thread about it somewhere...

Also, Von has new master cylinders available... Pics looked good!

ACAD_Cowboy · 11-06-2009, 04:49 AM

And I've also heard that Von is either

A. Blood thirsty parts vulture pedaling old used crap for NOS prices.

B. A pretty decent guy who is looking to make his parts business more than just a charity.

I'm going to worry about what I get from who once the truck is in my own driveway and is officially my own problem. Right now all I know is it has issues and every issue is a write down off the sale price.

But I will so note that supplying shop air in place of the OBA compressor will trouble shoot.

ACAD_Cowboy · 11-06-2009, 06:46 AM

Re-reading I'm not sure I was clear.

With the air over hydraulic brakes, if the air system is not charged should the brake pedal feel:

steady lack of pedal feel through whole range of travel

or

lack of pedal until late in travel with small build in resistance

Reading through some of the other posts it looks like the air system for the trailer coupling crosses over the air system for truck brake assist. Based on the manual this is to allow partial engagement of the trailer braking system for hill descent and for the e-brake. It appears however that no air in the system should lead to almost no pedal feel.

I'm going to have to catalog what is hanging under the truck to figure out which air assisted hydraulic system is has because while the book says the 404.0 is one way and the 404.1 is another, I don't recall seeing some components where it says they should be.

11-04-2009, 05:07 PM	#1
ACAD_Cowboy Registered User Join Date: Oct 2009 Member # 143944 Location: Hudson Valley New York Posts: 42	404.115 Air over Hydraulic Brakes With the air over hydraulics braking system, assuming a fully bled down air system, what should I expect to feel in terms of pressure at the pedal? Long story short I'm going to be spending a lot time with 404 I shared with you guys, doing some freelance work for it's owner. So I had some time to kill and wanted to try to develop a better idea about the truck, perhaps get the hood open and check the systems. So I got in the cab and checked the various pedals and levers. Throttle is pretty NFU due to carb gumming but the pedal travel is okay, same with the hand throttle. Clutch feels good, no drag or excessive force (compared to my diesel VW it's a monster but compared to my CJ it's about normal) and that leaves the braking system. The transmission is floating somewhere between this way and that way and most likely between gears and until I can spin the input I'm not going to try to force a gear. I suspect that to move it around someone discovered that if you put the direction selector in N and the gearbox in N it rolls around on the wonder tires (that never ever leak ever) without making funny noises. The brake pedal however seems to have very limited resistance. I didn't feel much of anything so I did not try to pump the system up. The E-brake has some feedback but I suspect the shoes are crusted in place. On the pedal, does the system require the pneumatics to be on line to function worth a damn? Do I just assume the system to be a write off and prepare to replace and rebuilt from the pedal back?

11-05-2009, 01:12 AM	#2
Public_AenimA Registered User Join Date: Mar 2008 Member # 110069 Posts: 91	It should function like any other assist system. With no air, it should be rock hard. I would plan on replacing or rebuilding all of the hydraulic parts of the brake system. Last edited by Public_AenimA; 11-05-2009 at 01:12 AM.

11-05-2009, 09:46 PM	#6
bigmellon Registered User Join Date: Nov 2007 Member # 102757 Location: Michigan, USA Posts: 56	You can make a fitting to fill the air tank from a shop compresor. If you do this and air gushes from the master cylinder... You need to get it rebuilt. Be careful, I rebuil done and the new parts didn't fit the best... Do some searching on brake issues here and on the benz forum and you can find a lot of data. My 404.0 was a royal pain and there is a thread about it somewhere... Also, Von has new master cylinders available... Pics looked good! __________________ - Kyle

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

11-05-2009, 08:05 AM	#3
ACAD_Cowboy Registered User Join Date: Oct 2009 Member # 143944 Location: Hudson Valley New York Posts: 42	Well you win some and lose some I guess. I find myself wishing this guy took better care of it versus just letting it biodegrade. Is it worth it trying to pump the system up or am I risking massivly screwing the master cylinder?

11-05-2009, 11:09 AM	#4
Public_AenimA Registered User Join Date: Mar 2008 Member # 110069 Posts: 91	All the hydraulic leaks I found in my system were from pitting on the bottom of the cylinder. Just little rust spots. It is already broke... how much worse can you make it? If the bore is pitted it is probably time to replace the master internals anyway. The master rebuild I bought from Eurotech was about 200$ and replaced everything but the casting. Then I had to hone the piss out of my cylinder but it worked when I was done. I few hours honing seemed better that 600$ for the new complete Master.

11-05-2009, 02:25 PM	#5
ACAD_Cowboy Registered User Join Date: Oct 2009 Member # 143944 Location: Hudson Valley New York Posts: 42	Well as I mentioned, the hood is caught up so for all I know it has a down stream leak and is dry. Until I can really crawl all over it it's all just guess work. On the related note, I penned a deal today with it's owner to do some freelance work on an extended basis so I'll have a lot of time with the truck to very carefully inspect it and not have the questions of just WTF am I doing. Now it'll be more like oh hey, how ya' doing. Getting the hood up and looking at fluid systems are my short term goal. And who knew you could barter for a unimog?!

11-06-2009, 04:49 AM	#7
ACAD_Cowboy Registered User Join Date: Oct 2009 Member # 143944 Location: Hudson Valley New York Posts: 42	And I've also heard that Von is either A. Blood thirsty parts vulture pedaling old used crap for NOS prices. B. A pretty decent guy who is looking to make his parts business more than just a charity. I'm going to worry about what I get from who once the truck is in my own driveway and is officially my own problem. Right now all I know is it has issues and every issue is a write down off the sale price. But I will so note that supplying shop air in place of the OBA compressor will trouble shoot.

11-06-2009, 06:46 AM	#8
ACAD_Cowboy Registered User Join Date: Oct 2009 Member # 143944 Location: Hudson Valley New York Posts: 42	Re-reading I'm not sure I was clear. With the air over hydraulic brakes, if the air system is not charged should the brake pedal feel: steady lack of pedal feel through whole range of travel or lack of pedal until late in travel with small build in resistance Reading through some of the other posts it looks like the air system for the trailer coupling crosses over the air system for truck brake assist. Based on the manual this is to allow partial engagement of the trailer braking system for hill descent and for the e-brake. It appears however that no air in the system should lead to almost no pedal feel. I'm going to have to catalog what is hanging under the truck to figure out which air assisted hydraulic system is has because while the book says the 404.0 is one way and the 404.1 is another, I don't recall seeing some components where it says they should be.