air over hydraulic brakes need explanation

blisters13 said:

Another good source of brake system components and theories:

http://www.mpi.mb.ca/PDFs/AirBrakeManual/ABM_Section2.pdf

-Ken J

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Excellent article. Even discusses the air over setup on a vehicle like what the deuce has. most appreciated!

tommys2patrick said:

Excellent article. Even discusses the air over setup on a vehicle like what the deuce has. most appreciated!

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That is the newer version of the manual I posted here years ago. It should be a must read for anyone with a big truck

tobyS said:

If you have the money, replace it with new or totally reconditioned. Internal rubber diaphragm elements dry rot. Water is your enemy where rust has eaten metal away. You'll find out when you have it apart...if there are elements that need replaced. In the case of air over hydraulic on a trailer, I see that was not the topic. Find new if it's available. My $02.

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I wished I did, but buying the truck, a bed and 11 G177 11.00x20 tire/wheels has already set me into close to $5k range and I have still many more miles to go before this project is put to bed. I see that NOS units are close to $600 so really don't want to go there if I can avoid it. Are the brake hydraulic/air assist units usually rebuildable? What would likely happen to one where it was not due to sitting? Internal rust as you mention but are these not sealed?

Now to ask, if the assist unit is bad but everything else is good, then I should have a very stiff brake pedal and even have working brakes albeit very stiff and hard to stop the truck if is going any speed. My having no pedal right now is more indicative of that there is either a bad MC but is there not a kind of second slave MC in the assist unit as well that could be bad? I sure wished I could get up to my truck and check out the MC and see if it is dry and if there is a telltale sign of brake fluid leaking from a wheel cylinder.

Lastly, who is the best seller of brake parts for an old 5 ton like mine?

No pedel means no fluid, fluid has to go somewhere, CKECK WHEEL CYLINDERS, FILL the mc, pump, pump, pump then LOOK on the back of the wheels, at the backing plate, LOOK for a LEAK, if no leaks THEN look at the FRONT of the mc.

there is NO secondary mc., think of the brake system as the same as your 53 chieftan ONLY BIGGER

I certainly will check all of those when I get back to my rig but there is a chamber in the air booster that holds brake fluid as well for air pressure to push against...correct?

PM sent

so yesterday I was able to purchase one of the old USGI brake pressure bleeding tanks from eBay but it does not come with the attachments. I am assuming from photos I see that there are no really special fittings in the kit and besides all I need is to be able to adapt to my own particular master cylinder anyway. My question is where do I find the specific instructions for using one of these bleeders? I see them shown in my truck's TM for maintenance level but it says nothing on how the tanks actually work? I assume the bleeders are just a pressure tank which has brake fluid in it and must just use stored air from a compressor? I expect the tank to be waiting for me when I get back so I can do a thorough bleeding of my truck's brakes and see if I can restore them to service.

steelcaptain said:

so yesterday I was able to purchase one of the old USGI brake pressure bleeding tanks from eBay but it does not come with the attachments. I am assuming from photos I see that there are no really special fittings in the kit and besides all I need is to be able to adapt to my own particular master cylinder anyway. My question is where do I find the specific instructions for using one of these bleeders? I see them shown in my truck's TM for maintenance level but it says nothing on how the tanks actually work? I assume the bleeders are just a pressure tank which has brake fluid in it and must just use stored air from a compressor? I expect the tank to be waiting for me when I get back so I can do a thorough bleeding of my truck's brakes and see if I can restore them to service.

View attachment 442296View attachment 442297

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You will probably be better off to build one of these ; Deuce Power Bleeder R4x4 Style

steelcaptain said:

I assume the bleeders are just a pressure tank which has brake fluid in it and must just use stored air from a compressor?

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That's right. I would bet money that unit came from Snap-on or was rebadged. I have one that looks similar. I think the older ones looked like yours.

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I've seen the TM for it but I can't remember where.

Snap-on appears to have discontinued it (I can't find it on their site) but you should be able to order hardware from them:

http://buy1.snapon.com/catalog/tools.asp?tool=all&Group_ID=675699&store=snapon-store

Notice the similarity in your parts and the parts on the snap-on page.

porkysplace said:

You will probably be better off to build one of these ; Deuce Power Bleeder R4x4 Style

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well sunnuvabichi...that's the way to go! I like that type a lot better and just sent an email to the seller of the one from eBay cancelling the purchase. I hope he gets it before the thing ships.

double sunnuvabichi...too late!

brake pedal pressure gone after winter parking air pressure ok

Started a new thread, then thought it would be better continue this since it deals with similar problems.

Subtitle says it all. Started to work on the truck, build up air pressure, pushed the brake pedal: nada nic nothing zilch. Like the pedal is not connected to anything anymore.

The system was flushed, all brake lines replaced and properly bled a few times in June 2011. Brakes were ok when parked here in June 2012 (16 months ago), and when I moved the truck back and forward a few yards in November 2012 when topping up the cooling fluid. Now nothing.

I do have a crack in the air line to the front connector for trailer or towing and I can clearly hear air escaping. However, that crack I already knew about last year about and it emptied the air tank quickly but should not influence the braking like this.

The puzzling thing is that the truck was parked with 100% functioning brakes and now suddenly is without anything. No visible oil leaks below the car, but it is standing on grass so if there was a big and quick leak when parking, 18 months ago, the signs probably would be gone already.

I had to drive the truck back and forward a few yards again to get it onto some wood prior to raising the truck and change the tires and check on the steering. Quite a problem to do that without brake. Even knowing I had no brakes I still got this empty feeling in my stomach when the truck overshot the wood and I automatically tried to brake (nothing to hit there anyway, but still).



I will get back to the truck today to work on the steering, and will look into this as well, check again, including fluid levels. Probably a faulty MC but any preliminary ideas how this can happen with a parked truck??

is there ever any mention how much fluid PSI the m35a2 generates as it hits the brake switch. if the wire to the old style sensor is damaged is it polarity sensitive and does the inner braided wire have any impact on putting a new connection. though I keep a switch in the tool box as backup would a 60s or 70s ford auto switch stand up to the dueces pressure.