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Gareth

Air over hydrolic brakes fluid change

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Hi all. Am wanting to have a play on getting my simca ready for a few days out and would like to give the lass a brake fluid change. Having a world of bother trying to find info on air over hydrolic brakes? They do work well at the mo but would like to make sure that no moisture has got in system. (Was told in can be an issue with them). Also would like to replace master cylinder seals if i can find any?(34mm bore) some of you may know the simca it was on ebay not long back and i lept at her. Has got a tdi 200 land rover lump with a 300 inlet and turbo married up to transit gearbox. Bit flat on bottom end but this will be going. Anyway any help, links or advice with braks would be appreciated

Edited by Gareth
Didnt read what put befor hittin send. 7 mth old baby had my attention

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When changing brake fluid I just use a pressure bleeder that puts fresh brake fluid into the reservoir. Open each bleeder in turn keeping about 5 psi in the system, and catch the old fluid in a can. Your system sounds like Air over Hydraulic.

Edited by john1950
change wording

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Thanks for replying john. I had a go other day with my pressure bleeder. And could only get fluid from master reservoir to the unit in front of air tanks, (dont know the correct name for the unit) where the air pressure is applied to system. Cant seem to get the pressure i would expect out of slave cylinders. So checked nips and all good. I have never worked on a system like it and am at a loss at mo:embarrassed:

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Gareth

 

I'll see if I can convert the diagram from the manual into a form I can post here. The way it works is that it is basically a hydraulic system with master and slave cylinders so if the engine is off and you depress the brake pedal it operates the master cylinder in the engine compartment which in turn operates the slave cylinder below and behind the driver's seat - the slave cylinder in turn forces oil into the wheel cylinders, but it is very very heavy to operate in that mode (there must actually be a bypass of the slave cylinder as the system can be filled only from the reservoir above the master cylinder).

 

If the engine is running the compressor charges the air tank to 6-8 atmospheres pressure in around 40 to 60 seconds, and when the air system is pressurised there is a valve operated by movement of the slave cylinder which releases air into a piston above the slave cylinder which assists the brake application. If working properly light pressure on the brake pedal is sufficient to trigger the air and the brakes are then applied fully and immediately. The air tank should hold enough pressure for 2 or 3 applications of the brakes for at least a fortnight after the engine is switched off.

 

The main issue I had with mine was leaking seals in the wheel cylinders which aren't very apparent until the brake drums are removed and the fluid traces are visible - fortunately they appear to be a common Bendix type (the SUMB replaced american vehicles in French service and I guess they wanted common spares) and they appear to be the same (from memory 34.5mm diameter) ones used in some American vehicles - seal kits and spare wheel cylinders for the M35 REO 2.5 ton truck are the same size.

 

Two other things to note from my experience

 

1) the brake lights are operated by a pressure switch on the back end of the slave cylinder which is badly exposed to mud, water and salt from the road. If it fails (mine did) you can't replace it except by letting fluid out when it is removed. I fitted a microswitch on the pedal rather than do that ! It also only operates if the slave cylinder achieves full travel (which only happens when air assisted unless you stand on the pedal) so is very much a signal of success in braking rather than the intention to brake!

 

2) The hand brake is not very effective and it is very easy to drive off with it applied. The hand brake is a rod operated drum brake on the back of the transfer box. If this happens it will both wear the brake shoes and get the drum very hot - this produces enough heat to boil the oil in the transfer box and blow it past the seals on the input shaft to the transfer box after a few miles - this is very messy and needs the drive shaft disconnecting to replace the seal.

 

I think you may find it helps with filling to cycle the slave cylinder by repeated applications of the brake pedal - I had to move one that had drained out its fluid via a wheel cylinder seal over 2 years of abandonment and I was able to get enough fluid into the cylinders that way to move and drive it on a recovery vehicle after topping up the reservoir and pressing the pedal a lot of times.

 

Hope this helps

 

Iain

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Hello Gareth. I think it sounds like a split system. Lots of looking and working out pipework first. If you have patience someone will have done it and be able to give you tips. As they say, the information is out there all you have to do is let it in.

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Diagrams from MAT4155 (the French Army service manual) attached

 

128.jpg

 

PS I don't speak French - Google Translate is not great at technical translations so you would be best to find a native speaker !

119.jpg

120.jpg

121.jpg

122.jpg

123.jpg

124.jpg

125.jpg

126.jpg

127.jpg

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Sorry for slow reply Iain. Had a gas leak in house! and the day as just run away. Am going to have a good read now and thank you very much for taking the the time to reply with what looks like a sterling effort. My missis can speak French quite well but she couldn't read the instructions for the transfer box. So don't think she will be able to help with this unfortunately. French auto mechanic needed i think for that job. Hope i can buy you drink at some point as a thank you. If i can get a full translation i will post it for anyone who may need if thats ok

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Gareth

 

I think the French automotive vocabulary is a separate subject to GCSE French ! I got the pages from a public French website many years ago (r2087.free.fr I think) and had some help from a then colleague who was French and had done national service - any copyright probably belongs to the French Government (their MATs are equivalent to our EMERs and AESPs) so I think it would be safe to post a translation

 

Regards

 

Iain

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Just to add i have already checked all wheel cylinders and no leaks. Which was reassuring. I drove the old girl home when i bought her (over 80 mile) had taken recovery vehicle but wasn't needed :cheesy:. The brake light switch was a little dicky so have already fitted a switch to pedal. Can you remember if you had to run engine when bleeding? And thanks again much appreciated. Duly noted

Edited by Gareth

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Have also found a small leak on dif lock. The leaver in the cab is leaking but dont think that will be to bad to sort. Hope its not a axel split job if last owner was trying to use with leak

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You shouldn't need to run the engine as the hydraulic system is isolated from the air system and works as a pure oil system with the engine off. If you have a pressure bleeder I think it is enough to hold the pedal down - if you are working by keeping the reservoir topped up and pumping via the pedal (as I had to) it may make the work of pressurising the pipes to the wheel cylinders easier if there is air assist though!

 

I've been going back through my files.

 

There is a list of Bendix Air-Pak type numbers at: http://precisionrebuilders.com/pdf_catalogs/VacCatalogPHB-0398.pdf page 80

I think the SUMB uses the same Bendix parts as the American M35 2.5 ton truck in the main although the air-pak (SUMB looks more like the 5 ton US Army one) and master cylinder (the M35 one has an integrated reservoir) are not identical the operating principle of the single circuit air over hydraulic brakes is the same !

 

If you can identify the Bendix part number for the Air-Pak then it is quite likely that you can still get a rebuild kit for the equivalent US Army part (and a decent drawing from their technical manual!) from American e-bay sellers.

 

The very similar M35 Reo system is described from page 304 onwards of http://www.jatonkam35s.com/DeuceTechnicalManuals/TB-9-2300-426-20.pdf - they suggest that the air-pak (and implicitly the pipework from the reservoir through the master cylinder to the air pak) is bled first followed by the wheels, starting with the one furthest (in terms of pipe length) from the Air-Pak which will be Right rear I think.

 

A couple of things to test

 

1) There are pressure relief valves under the air cylinders which should release air if pushed in from below (use a metal tool not a finger!) - looking at Fig. 58 I think the rear air tank feeds the diff lock and the trailer air brakes, whereas the front air tank feeds the air pak cylinder. So I would expect the diff lock lever air leak to only affect the rear tank and not the brakes if the non-return valve (No. 20 in the diagram) is OK.

 

2) If you press the brake pedal after the engine is turned off you should hear and feel the air cylinder firing after the pedal is depressed by 20%-30% - if the original brake switch was intermittent then there may be a lack of fluid at the brake end of the slave hydraulic cylinder of the air pak - this may be helped by bleeding. If I remember correctly the brakepipes to the wheels come off the end of the piston tube near the switch so you may be able to disconnect and check hydraulic pressure there,

 

It is a single circuit system so it is necessary to be very careful as a leak can affect all four wheels and the hand brake is not strong enough to stop the vehicle (one of mine was so worn that it rolled off the transporter with the handbrake on!).

 

Iain

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You are 100% correct on the brendix air-pak. Might book her on to a rolling road after i have had a play. Just to give a little peace of mind. And the hand brake even after a little tinker is near pointless. You do seem to know them well. I am half way through putting a new clutch master cylinder with its own reservoir on. So at least if brake failure happens i can down gears. They do work well since bleeding reservoir to air pak but would like to get my head round the workings.

 

Also the air tank hold air for weeks as when i first bought the truck i forgot to drain tanks after running engine. Was left for around 16 days and when i jumped in and pressed pedal i could hear the air. And blow off works well as after 10 ish mins of running it goes with a good blast.

 

Will report back after a hopefully dry day of tinkering.

 

Thank you again Iain ......wish we lived a closer to each other

 

Gaz

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If the system has not been bled for some time are the bleed nipples clear?

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I did take one out when i first had a go at bleeding. But it is a possibility. Have checked the air pak and all seems well. Not sure how much air force/pressure should come ftom from the valve you said to press with a tool. But it dose release air when pressed. The blow off is working, maybe to much. Air pressure is running at 10 bar on tick over. Change of pulleys needed i think. Would helpnif it stopped raining. Also followed your link for manual but no joy. And my have a translator but will see if i can get full manual first

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Gareth

 

10 Bar is good - my best SUMB achieved that.

 

The manual seems to be now available as a PDF at: http://armytrucks.free.fr/cariboost_files/tm/MAT4155.pdf - Brakes ("Freins") start from PDF page 114 / original page 118 onwards. The version I had was JPEG scans of individual pages but I can't find it six or seven years on !

 

Also the drivers manual http://armytrucks.free.fr/cariboost_files/tm/MAT2719%20.pdf

 

The site I remembered is http://r2087.free.fr/new/pages.php3?num=138 but I cant find the manuals there now

 

Sorry

 

Iain

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Please dont apologise Iain. You have been a massive help, not as much as I must be a pain in the ...

 

Next job will be to find a capable translator. I have been trying different translation sites but it is its own language.

 

i must have stopped your life for last few days sorry.

 

To give you an up date tho. Clutch took way to long and then after a good power wash found an oil leak on compressor. But after a strip down and rebuild got the 10bar on tick over. Hope its not working to well on the diesel engine. And made an improvement on throttle mount.

 

So brakes will be tomorrow, but its looking good.

 

Can't thank you enough Iain the great

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