Air over hydraulic brakes ?

[R Cubed]

I'm sure a person as versatile as the Cubed one would have no issues in finding homes for the two airpacks, twin master cylinders, sensing tank and regulator plus routing all the hydraulic lines, as well as the low and high pressure airlines..

 

Yes there is lots of room in a ben hur trailer, all that lot could fit in there :nut:

[R Cubed]

Well diving off in to the realms of stupidity now. How about this set-up.

 

You have a full air system from foot brake pedal right to the wheel actuators, but rather than these operating the " S " cams on the brakes, the actuators operate air over hydraulic slave cylinders one for each axle so 3 in total these would then run down to the existing truck wheel cylinders. The problem of the air / hydraulic units not having enough hydraulic fluid volume would be ruled out as there would be 3 separate hydraulic circuits no chance of total brake failure and a reliable air system for the foot pedal.

[ArtistsRifles]

Joking aside - it sounds a complex and expensive set-up for a GMC... No offence but I thought they were meant to be pretty basic, easy to fix vehicles? What you describe takes it way outside of that remit.

 

Another thought - you improve the brakes, OK - how good are the tyres in terms of grip? The last thing you want is all 6 locked into a skid because the brakes are now too good. I've had that once in the Stolly and believe me it ain't fun!!

[Stormin]

Joking aside - it sounds a complex and expensive set-up for a GMC... No offence but I thought they were meant to be pretty basic, easy to fix vehicles? What you describe takes it way outside of that remit.

 

Another thought - you improve the brakes, OK - how good are the tyres in terms of grip? The last thing you want is all 6 locked into a skid because the brakes are now too good. I've had that once in the Stolly and believe me it ain't fun!!

 

I would say you do want to be able to lock all six wheels, if the vehicle is unladen and you hit the pedal and can't lock the wheels up something is wrong. It's then down to the driver to mitigate wheel locking by skilful driving according to load and grip conditions.

 

I'll agree with Neil though that the proposed air to hydraulic link at each wheel sounds overly complicated.

 

Sounds like you've already spent quite a bit of money on the current brake set-up, why bin it all in favour of another experiment? You mention that most of the mechanical parts are new or overhauled but what about the friction surfaces? Have you had the brake shoes re-lined and with what compound, are the drums new or skimmed?

 

Brake linings come in many different grades, with differing initial grip / bite from hot and cold. Modern lining materials designed for heavy trucks with high power ABS brakes will not really suit your old hydraulic braked truck. They are also designed for high mileage and high wear resistance therefore are a harder compound giving lower initial grip. May well be worth trying some different brake linings.

Also what is the condition of the drums? Have they been skimmed are they still within tolerance? Even if they have been skimmed past oil contamination can still be soaked into the cast material.

[ArtistsRifles]

I would say you do want to be able to lock all six wheels, if the vehicle is unladen and you hit the pedal and can't lock the wheels up something is wrong. It's then down to the driver to mitigate wheel locking by skilful driving according to load and grip conditions.</snipped>

 

 

I was thinking that the set-up proposed stood a fair chance of doing what a Citroen DS was known for with an inexperienced driver - locking up as soon as you touch the brake pedal. Getting all 6 locked might be a good thing if you've stamped on the pedal as though your life depended on it - getting all 6 locked by inadvertently pressing too hard is not so good..... :-(

[stal108]

There is an option, i have a big american chevy camper its brakes are power brakes ie run off the pas pump to assist easy set up, You can see in the pic the red caps at the rear of the master cylinder this is where the feed is the other red cap is blanked off the return is the pipe on top.

Master cylinder is just that no big deal, what do you recon????

[R Cubed]

I was thinking that the set-up proposed stood a fair chance of doing what a Citroen DS was known for with an inexperienced driver - locking up as soon as you touch the brake pedal. Getting all 6 locked might be a good thing if you've stamped on the pedal as though your life depended on it - getting all 6 locked by inadvertently pressing too hard is not so good..... :-(

 

Surely the pressing too hard would be down to the experience of the driver, learning how the vehicle reacts in relation to how fast and how hard you push the pedal and also the feedback of the pedal as you push it. I have driven some really new cars while my works van has been in for servicing and repairs ect and new cars brakes are awful, so light and no feel as soon as you touch them you are through the windscreen, well it feels like that.....

[R Cubed]

There is an option, i have a big American chevy camper its brakes are power brakes i.e. run off the pas pump to assist easy set up, You can see in the pic the red caps at the rear of the master cylinder this is where the feed is the other red cap is blanked off the return is the pipe on top.

Master cylinder is just that no big deal, what do you reckon????

 

That's a thought, I have a hydraulic powered master cylinder off a BMW 735csi operates just as you described runs off the power steering system.. Food for thought.

[R Cubed]

Joking aside - it sounds a complex and expensive set-up for a GMC... No offence but I thought they were meant to be pretty basic, easy to fix vehicles? What you describe takes it way outside of that remit.

 

Another thought - you improve the brakes, OK - how good are the tyres in terms of grip? The last thing you want is all 6 locked into a skid because the brakes are now too good. I've had that once in the Stolly and believe me it ain't fun!!

 

This is all in the thought process thanks, if the truck is loaded and we need to stop it quickly it would be nice to know that the brakes could lock it all up if needed, also being loaded I am not so keen on the only single circuit bit, although I regularly inspect the whole system on my truck you don't really know the quality of the weakest bits, which in my view would be the flexible hoses to the wheel cylinders. Only need one to burst and you right in the doo doo....

[R Cubed]

I would say you do want to be able to lock all six wheels, if the vehicle is unladen and you hit the pedal and can't lock the wheels up something is wrong. It's then down to the driver to mitigate wheel locking by skilful driving according to load and grip conditions.

 

I'll agree with Neil though that the proposed air to hydraulic link at each wheel sounds overly complicated.

 

Sounds like you've already spent quite a bit of money on the current brake set-up, why bin it all in favour of another experiment? You mention that most of the mechanical parts are new or overhauled but what about the friction surfaces? Have you had the brake shoes re-lined and with what compound, are the drums new or skimmed?

 

Brake linings come in many different grades, with differing initial grip / bite from hot and cold. Modern lining materials designed for heavy trucks with high power ABS brakes will not really suit your old hydraulic braked truck. They are also designed for high mileage and high wear resistance therefore are a harder compound giving lower initial grip. May well be worth trying some different brake linings.

Also what is the condition of the drums? Have they been skimmed are they still within tolerance? Even if they have been skimmed past oil contamination can still be soaked into the cast material.

 

Sorry forgot to mention this is not for my 352 at present but for a hypothetical 353 being used loaded (upwards of 3 tonnes) on road.

 

Going back to my truck when I did the brakes about 10 years ago, I overhauled all components and relined all the shoes with lining from Rex Ward so should be up to the job, theses are now getting low in places and so will soon be due a change, as will the hydrovac will have a strip down and a clean out and check over, don't have new drums ( probably not available ) they have not been skimmed as they are in good condition, not scored or damaged. Would be interested in the tollerance limit dont know if anyone would know it, if someone does please let us know ?

[R Cubed]

Testing and messing about with the brakes. Thinking out loud, now how can I prove a trial system works, if I measure the wheel cylinder pressures of the original system at a given force on the brake pedal, say by putting some scales between my feet and the pedal and apply a set force then read the pressure ?

Then fit a new type system and do the same test and get a better reading with less force would it be reasonable to assume the brakes would be better ?

 

Also the brake bias would not be affected as the wheel cylinders will still be the same so the effective forces will be the same in relation to front and back.

 

This is assuming the volumes of fluid are equal to what has been removed.

 

Also please note these trials would not be carried out on the road.

[Stormin]

Testing and messing about with the brakes. Thinking out loud, now how can I prove a trial system works, if I measure the wheel cylinder pressures of the original system at a given force on the brake pedal, say by putting some scales between my feet and the pedal and apply a set force then read the pressure ?

Then fit a new type system and do the same test and get a better reading with less force would it be reasonable to assume the brakes would be better ?

 

Also the brake bias would not be affected as the wheel cylinders will still be the same so the effective forces will be the same in relation to front and back.

 

This is assuming the volumes of fluid are equal to what has been removed.

 

Also please note these trials would not be carried out on the road.

 

Well since the system is single circuit you could measure fluid pressure straight out of the master cylinder. You could also work out pedal force, pedal lever arm and master cylinder diameter and see how much assitance the hydrovac is giving above the theoretical pressure.

Do you really want to put extra hydraulic pressure to the existing slave cylinders, possibly above what they were designed for?

[R Cubed]

Well since the system is single circuit you could measure fluid pressure straight out of the master cylinder. You could also work out pedal force, pedal lever arm and master cylinder diameter and see how much assitance the hydrovac is giving above the theoretical pressure.

Do you really want to put extra hydraulic pressure to the existing slave cylinders, possibly above what they were designed for?

 

No dont want to increase the pressure but make the system safer as only single circuit and go to dual circuit. The idea of finding out the pressure before the hydrovac and then after it is an interesting one did no think of that one.