| Re: DIY - Simple Hand brake for Jeeps - Ball Valve as a Line Lock
Since I'm a newbie, will introduce myself - Erik, mechanical engineer formerly working in the U.S. in the remanufactured auto parts industry. We did just about everything from brake/steering hydraulics to ABS systems, engine control modules, and electric motors (window lift, wipers, blower, etc). I did hands-on work / R&D / test machine design, etc, etc, for most of these lines at one time or the other...
IMHO, there is really not going to be much issue with accelerated wear of wheel cylinder / caliper components. There are plenty of seals in all kinds of industrial hydraulic components that are under full pressure constantly - even the simplest o-ring is designed to deform from its original shape under pressure, and to return to it when off pressure. That is actually the mechanism for its sealing under higher pressures. Wheel cylinder cups are really no different. Most wear / failure on hydraulic systems is going to have more to do with water / sand / grit entry and related rust, etc, getting in there. Or for that matter, it is just the grit / sand / dust that the mechanics themselves leave inside the system when they "rebuild" whichever compenent (in my observation, there are almost never any clean shop rags to be found among desi mechanics, and hands are only washed when it's time to go home. I have yet to see a proper parts-washing setup, too, though it would actually be very easy to make one). Secondly, rubber hardens with age and can't deform / reform as quickly / easily (or at all), causing leaks. Up to that point though, IF YOUR FLUID IS CLEAN, the number of actuations possible before failure, and the amount of time spent under pressure vs. without, has virtually no bearing on life expectancy.
I think the setup described / shown in the original post is perfect as a sort of "hill-holder" - just makes it easier to get going again when you can focus on clutch & accelerator with your feet and release the brakes by hand. Toe-and-heel can be a pain, especially if your pedals are not well-placed. This solves that issue brilliantly.
Re: parking duty, it's a valid concern re: holding time. Since hydraulic fluid is only minimally compressible, any tiny leak in the system (whether internal at the ball valve or external in the lines / wheel cylinders) will gradually reduce the pressure to whatever brake you're trying to lock, and it will eventually release. A ball valve HOLDS pressure, but doesn't have any ability to add any to make up for any small losses. What is really needed to eliminate this problem / risk could be the addition in the line (between the ball-valve and whichever set of wheels you're locking) of something like a small clutch master-cylinder actuated with an "over-centering" type clamp / lever - the latter stores a bit of mechanical energy that could continue to push / create additional pressure even if the pressure in the line was slowly seeping out someplace. The school buses I rode as a child in the U.S. used something like this - being very long vehicles, cables would not have been effective, and safety / reliability was obviously a top priority. A medium/heavy compression-type spring could be added between the clamp and this auxiliary MC that would provide even more "storage" of linear force (and thus hydraulic pressure), if so desired. Other option - really better if you can find / make one, would be what's called an "accumulator" and add that to the same place in the line. They're used in all kinds of hydraulics to store pressure, absorb pressure spikes, etc. Put a little one (say, one that would hold a couple ml's at a few hundred psi) in a T somewhere between the ball valve and your locked wheels, and basically, it will absolutely hold firm, until more than 2ml's of fluid has leaked from the system someplace (which is not very likely to happen - and if it did, you've got a lot more serious safety problems in your braking system than any lack of a parking brake!).
Last, while creating a sort of "locker" setup hydraulically with ball valves sounds appealing, it's difficult and I can't see how one could really make it work practically. Again, ball valves can't create pressure, only hold it - so if one wheel starts spinning, how are you going to apply pressure to just that wheel? I suppose you have in mind to lock both (or all 4) wheels first by hitting the brake pedal and closing four ball valves, then releasing pressure to whichever wheel(s) were stuck by opening two of the ball valves? But then you've also got to release pressure to whichever wheel was spinning immediately, too, or else the drag those wheels is creating by being locked is going to impede forward motion and probably get the two useful wheels you had pretty well buried, too.
Before they had differential locks, many old tractors had mechanical brakes on the rear wheels operated separately by manual levers. The idea was that if one side started spinning, you could sort of "jog" the lever for a split second - or else apply steady but only partial force - to get the other one moving you forward. Any more than a split second or partial force, though, and the tractor goes wild / gets stuck worse. So if you're going to do this hydraulically, I guess you'd need to have the brake pedal up, close all your ball valves (no brakes now at all - scary???), hit the accelerator and see which wheel(s) are spinning, then open the ball valves for those wheels only, hit the accelerator AND at the same time, jog those brakes with the brake pedal, which is now only applying pressure to the spinning wheels and transferring that rotational force to the wheels with more grip... Doesn't sound very easy and can't be done while in motion, since you have to lock all the wheels initially.
Other option would be to have 4 ball valves / 4 hand-operated auxiliary master (clutch-type) cylinders, one in each wheel line. Get stuck? Fine, close all your ball valves (so that when you activate any of the individual wheel master cylinders, it doesn't just push the fluid back up to the main master cylinder). Then hand-operate the 4 individual levers as needed to stop whichever wheels from spinning. Easier to jog / apply part pressure this way. But hydraulically a little daunting with all the extra lines / seals / controls, and probably not that easy to use, either.
This sounds confusing, maybe? Makes perfect sense to me and very possibly to nobody else... sorry if so. It could all work, of course, but seems hydraulically too complex (FIVE master cylinders and four ball valves on ONE car???), and besides being pretty tricky to operate, it seems very likely unsafe, too, since if at any point you happen to forget that your ball valves are shut and suddenly need your brakes and go for the pedal instead of the hand levers, well, you're in trouble... Then there are the aforementioned maintenance issues. Lots more stuff for your mechanics to get grit into.
Incidentally, in places like S.P. Road (B'lore) and Ajmeri Gate (Delhi) there are many, many shops dealing in good quality imported industrial hydraulic components from where you can get ball valves that most definitely will not leak any time soon. Ball valves by design are some of the best-sealing types of valves, and assuming your brake fluid is clean (you should flush the system at least once yearly and not let any mechanic introduce new grit into the MC when he checks the fluid level or whatever, or into the wheel cylinders / lines during any other service) you are unlikely to have any problems.
As a parking-brake / hill-holder, I think the single ball-valve with a pressure accumulator in the line would be perfect. Safe, strong, and every bit as reliable as the rest of the brake system. A hand-brake for emergency-type situations really is unnecessarily redundant if you have a "dual" master cylinder (I think Mahindra must have started using these pretty far back) whereby the front / rear systems are operated on separate internal hydraulic circuits anyway. Hence, any loss of pressure in either the rear or the front system still allows the other half to work. You've just got to put your foot a lot further down towards the floor. Lots of people don't know this. The pedal goes down further than usual and they think they've totally lost their brakes and crash unnecessarily. Not so. Put it all the way down. There'll be pressure down there somewhere (usually about an inch off the floor)!!! Anyway, not having a hand-operated "emergency brake" is not a "CON" in my book. They were true "emergency brakes" only back in the days of single-channel hydraulic braking systems, where any failure ANYWHERE in the system had you losing ALL pressure.
Enough (too much, really) for now. Have fun. | 
26th May 2012, 15:05
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Thanks
