Dual circuit hydraulic brakes

Because there are 2 cylinders in a dual circuit brake systems master cylinder, in normal operation the fluid between the 2 cylinders pushes the

when one of the circuits goes down, when you push on the pedal, the cylinder with no fluid in it moves more than the one with the fluid in it, at some point the 2 cylinders will touch mechanicaly, and the other circuit will get what presure's left from you pushing the pedal down.

If you were to try and move a wheel on the circuit that's not being bled, you should find it's locked when you have the pedal on the floor,

They do that so you know you have a problem and are braking on only half the system, you have reduced braking capacity, but should still be ablt to stop.. allbeit in a longer than normal distance,

I've had the master cylinder go on a beach buggy i had, (1971 VW beetle based) i lost the front circuit, the pedal when almost to the floor, but not quite all the way, and i was able to stop.. i was out playing in the snow when it happened, so i didnt want to stop quick anyway,

Also had a pipe rub through on the rear circuit of a 7.5 ton truck i drove, it had air over hydraulic brakes (are pressure controlled by the foot pedal pushes on a couple of diaphrams, that move a couple of hydraulic master cylinders.. from then on it's like a cars brake system)

With that, the brakes felt normal when stopping from slow speeds, but when i was doing 50mph on the A52 coming out of nottingham, heading to grantham, about half a mile from the traffic lights just before the radcliffe turning the lights changed, i pressed the pedal, and it felt like nothing was happening, i managed to stop on the foot brake about half a trucks length over the stop line for the lights, luckily the road was clear, and i didn't have to use the parking brake, as that used to snatch and wouldent release every time so i was reluctant to risk locking the rear wheels up, and having them stay locked while the road was clear (yes, i drove sheds back then, wasnt my truck either.. it'd scare you to know it was a recovery truck!)

We re-filled the rear circuit's master cylinder, and the brake pedal felt normal again, untill all the fluid was pumped out of the leaking line, then it felt slightly less responsive, but not in a way you'd notice.

BTW, in most cars and vans nowadays, the split is front and back, every vehicle i've owned is like this, loose a circuit, and you still have either the front 2 or rear 2 wheels with limited braking,

Not sure how an ABS equiped vehicle handles loosing a circuit, does it reduce power to the oposite wheel so you dont end up spinning the vehicle when you stamp on the brakes and find out you've lost the front right circuit only, so have double the braking effort on one side, and half on the other, or does the hydraulic pump empty all the fluid out of the system when one line goes down?

SNIP

No, they're mostly diagonally split nowadays, i.e. O/S front with N/S rear and vice versa. This is to prevent you having only front brakes working in which event the front stops and the back end comes round and overtakes it.

But I can't help with the ABS thing. Probably a computer stops the engine, locks the doors and calls the police to grass you up.

Steve

Surely ABS only stops the wheels locking, it doesn't reduce braking power to the wheel unless it's at the point of locking. Hence the braking would be more-or-less [spot the get-out clause!] the same as the non-ABS case, but with no risk of locking the wheels. Vehicles with Electronic Stability gubbins might be a different kettle of fish though.

Aye, thing i was thinking of is that most abs systems dont use a vacuum servo, rather a hydraulic pump and modulator, and have 4 seperate pipes from it to the wheels, where as non abs vehicles have a master cylinder with 2 cirsuits, and 2 wheels share a common feed line, so loose one wheel's flexi hose say, and you loose the braking on the other end of the circuit,

Thinking with abs, loose a flexi hose, and only one wheel looses braking, untill the modulator pumps all the remaining brake fluid out and you loose the lot, or it shuts down that line so you dont loose all the fluid,

this is all specultaion tho, not worked on an abs equiped vehicle for a while, and then that was hgv's with abs air brakes, where any leak and the spring brakes activate, and bring the brakes on,

RE: the diagonal verses front/rear split, i was basing that on what i've driven before.. but that has included 2 large vans for the past 7 years, where because of the load sensing valve, the brakes have to be split fron an d back, so the valve regulates only the braking pressure of the brakes on the rear wheels.

Mind, on the abs equiped vans (iveco turbo daily) the manual shows a complex double load sensing valve, and 3 brake lines to the valve for some reason, that's just goiung on the drawings of the system, i dont have abs on my van, so have a standard single line in, 2 lines out from the load sensing valve, with a few pressure test fittings before and after the valve thankfully, as i'll be fitting air suspension later on, and will need to adjust the valve, something that was a pain with the LT i last did this to due to no pressure take off points to attatch the gauge to easily.

As far as I know, non ABS is split as a triangle, each circuit serves one rear brake and half of the two front brakes, and ABS systems are split both front and both rear. Remember that the brake warning light should come on in the event of hydraulic circuit failure or at least a low fluid level warning, meaning that you shouldn't really proceed without checking things out.