97 SL320
You are describing a sequential operation when our brake master cylinder front and rear are acting simultaneously.


http://www.gomog.com/allmorgan/MC1.jpg

Dual-Circuit master cylinders don't operate "front and rear" of the car, they operate opposing corners (front left, rear right for example). The push-rod pushes on both circuits at the same time. There is a bit of "slop" in the fit between the first and second piston and the spring does take up the slop to an extent, but it isn't full-stroke. Only enough to allow some equalization between the first and second piston. If the link works, you can see a better cutaway:

You can't make that as a blanket statement.
Subaru splits their units diagonally...
but many cars are split front/rear .

Diagonal split is the norm rather than the exception. The whole idea being that if you lose one circuit of the braking system, you can still safely stop without losing control. Having only the front or only the rear working can very easily cause a spinout or rollover.

The pic you reference is from the link I posted, and it shows operation when the "front" braking circuit has a leak. In this case, piston 1 travels much farther then normal until it contacts piston 2. This would be considered full stroke of piston 1.

A leak in any circuit = full stroke of the remaining good circuit The MC better be in good enough condition to tolerate this, if the MC can't tolerate full stroke bleeding, it needs replaced.

I agree that if the unit has corrosion which messes up the seals if the movement is allowed to go too far by the person in charge of the bleeder nipple at the wheel it should be replaced..

Have you looked at the MC picture in the FSM ?
The use of the word ' much ' and ' full stroke' etc... are a problem.

Not strictly true...

A leak in the Secondary circuit (end of the MC) would not cause a "full stroke" of the Primary circuit (in contact with the brake pedal pushrod). The Primary circuit will move only the amount needed to stop the vehicle, although the pedal will be "soft" due to the lack of hydraulic pressure on the Secondary circuit. Depending on the health of the brakes and design of the braking system, the stroke of the Primary will be more than "normal" but less than "full" unless a panic brake situation is encountered.

Conversely, a leak in the Primary circuit won't cause that piston to move "full stroke" to contact the Secondary piston either. The recess in the Secondary is enough to allow a difference in pressure between the 2 circuits, but not enough to allow the Primary to move it's full stroke before engaging the Secondary circuit.

The only way to have a full-stroke movement in either circuit is to have a leak in both and move the pedal to full-travel position. This is essentially what a "bench bleed" does since there isn't any hydraulic pressure on the system. You can move both pistons to their full maximum stroke to burp all the air out before installing in the vehicle.

You are correct though. If the MC is so worn out that a "full stroke" or "high pressure" plunge damages it, it certainly wouldn't survive a "panic brake" operation either. If bleeding the brakes killed the MC (bleeding is considered routine maintenance), it was on borrowed time to begin with.

This isn't entirely true, the split method depends on the the cars drive system.

A rear drive has always been setup with a front / rear split.* This works because the rear wheels have a decent amount of weight on them and will stop the car well enough on their own.

A diagonal doesn't work so well in a rear drive car because the steering is generally set up with little / no negative front scrub radius. If one front wheel has braking and the other does not, the car will tend to wind the steering wheel in the direction of the operating front brake.

* 80's Volvos used a 4 piston 2 circuit front caliper to make a triangular split using 1/2 of each front caliper and one rear. This gave near full front braking and the imbalance in the rear was manageable.


A front drive needs a diagonal split. With so much weight on the front wheels, losing front brakes and relying on rears will result in locked rear wheels and round the car goes. A front drive car can tolerate a diagonal split because it has more negative front scrub radius, this tends to turn the steering wheel in the direction opposite of the operating front brake countering some of the braking imbalance.

Scrub radius on a double a arm type suspension is found by drawing a vertical line through the tire tread then drawing a line through the upper / lower ball joints. The distance between where the two lines meet the road surface is the scrub radius.

If the LF wheel is pushed outwards from the center of the car, the scrub radius is now positive. Road irregularities / surface condition / braking imbalance will tend to steer the car to the left.

If the LF wheel is pushed inwards from the center of the car, the scrub radius is now negative. Road irregularities / surface condition / braking imbalance will tend to steer the car to the right. This result is helpful in reducing torque steer in a front drive car.

70s and 80s Fords and GM's were diagonal. In that era, they were almost all RWD. Their systems used a proportioning valve to allow different braking pressures to the front and rear wheels.

Gee, I've been doing this all wrong for the 40+ years I've been in and around the auto repair industry. It's no wonder that all the cars I've worked on and race cars I've built don't work properly. If you think the VW TDI recall is massive, just wait until I generate my own recall.

On the cars you reference, don't confuse both MC lines entering a frame mounted combination valve with an actual diagonal system. This combo valve contains a piston and switch to warn of a brake pressure imbalance and a proportioning valve that reduces rear braking pressure in proportion to front pressure, this isn't a regulator where outlet pressure is constant.

On the rear drive cars you reference, do they have one brake line running to the rear or two? Assuming a solid rear axle, is there one rubber brake hose running from the body or two? ( Short whip brake hoses from the brake caliper to the axle don't count on rear disc cars. )

The attached pic from this link looks like it was lifted from the factory VW manual.

| Repair Guides | Brake System | Basic Operating Principles | AutoZone.com

I will give that while a circuit might not go total full stroke, it will travel significantly more than normal when one circuit develops a leak / bleeder is opened to the point it might as well be considered full stroke.

Here is an animated MC with leaks in various circuits + dual circuit layout considerations.

1PORTER AND CHESTER INSTITUTE. 2 Brake Hydraulic Systems. - ppt download

brakes
 

Ironandsteel after reading all the posts of the ins, and outs, of the brake system .Did you manage to get your brakes to work after all ?