A wilde ride home today, white knuckel

[SteveJ]

Don't forget the hose clips. They get old and rusty.

[LeonV]

As suspected, catastrophic failure of the rear left wheel cylinder. No warning, just let go. I put new pads and shoes on it last year and inspected the entire system before putting the car on the road. Just one of those things I guess. So what is our best source for new wheel cylinders?

Strange. A rear wheel cylinder blew out and you lost all braking power? This is the reason for separate front and rear circuits so a failure in one does not cause a complete loss of stopping ability. Front brakes do most of the work so you should be able to stop just fine without any rear brakes. My old Z managed to stop without worry even with a complete loss of the rear brakes. I didn't even realize at the time that I had no rear brakes until I popped the hood and noticed no fluid in the rear brake reservoir! Methinks you have another problem besides the wheel cylinder.

[5thhorsemann]

Strange. A rear wheel cylinder blew out and you lost all braking power? This is the reason for separate front and rear circuits so a failure in one does not cause a complete loss of stopping ability. Front brakes do most of the work so you should be able to stop just fine without any rear brakes. My old Z managed to stop without worry even with a complete loss of the rear brakes. I didn't even realize at the time that I had no rear brakes until I popped the hood and noticed no fluid in the rear brake reservoir! Methinks you have another problem besides the wheel cylinder.

Damn it Leon, I just looked at the FSM and you are right! the peddal goes rite to the floor and I can move the car with only one foot out the door while sitting in the drivers seat. Good fluid in the front res. but as I pump the peddal with no fluid in the rear res. fluid is draining from the front res. Looks like the master cyl. is also toast. Rear disks, here we go.

[5thhorsemann]

Ok, so I put a set of gauges on the master cylinder and they read as follows;

back= 740 psi

front= 720 psi

Therefor, I would have to assume the MS is good as there is no leakage at the booster and the pressures are different. Note that I isolated the MS from the calipers and wheel cylinders.

Looking at the FSM cross section diagrams, it looks like the front brake piston is actuated by the rear piston via fluid. So to test this I re-connected the front brakes, left the gauge on the rear, started the car and applied the brakes. The rear gauge maxed out at just over 800 psi and the front brakes worked. So it was just the rear cylinder failure that caused all the brakes to go away. I did have some brake pressure up front while driving home the other day, but it was not safe by any means. The peddal went all the way to the floor and the brakes only slowed the car a bit, no real stopping power without the hand brake.

So my question is, would this be correct or am I missing something?

[Zed Head]

After I read your story I thought the same thing that LeonV did and almost responded the same way. But, coincidentally, I had just bled my rear brakes so it was fresh in my mind that the pedal (in my car) will go all the way down if the back bleeder valve is open (similar to a blown wheel cylinder).

I also checked the FSM and came to the same conclusion about how the master cylinder works.

I wonder if the pressure differential valve is working correctly, on your car and mine? As mentioned, I too thought that it would isolate front from back if the pressure differential was too great, but my experience with bleeding the backs on my car, and your wild ride don't support that. Maybe our valves are not working right. I've searched the FSM but have not found where it says explicitly that it does more than make the Brake Check light go on.

I'm curious, if anyone knows for sure. It would be worth tearing things down to fix that valve if it is supposed to be a fail-safe device.

Edit - LeonV, I believe your story, just looking for more examples and insight. You might have had a slow leak that still allowed some pressure buildup(maybe?).

Edited March 31, 2011 by Zed Head

[LeonV]

That is typical tandem master cylinder operation. Thus, you will have a dead spot in the pedal for a set amount of movement before the primary piston actuates its circuit. However, there should be ample stopping force with only half the circuit working especially if it's the front half. It doesn't have anything to do with the proportioning valve, which varies braking pressure to the rear cylinders in order to keep them from locking the rears first.

In the bleeding example, the pedal will go to the floor if the fronts are not properly bled. If you have some time on your hands, re-bleed the rears with the fronts properly bled and you should have some pedal resistance.

5thhorsemann, when was the last time you bled your brakes?

Edited April 1, 2011 by LeonV
Clarification

[5thhorsemann]

I admit that I never have cracked the bleeders on this car, never opened the system (aside from my little test above) so no need to bleed in my mind. I'm thinking I need to look at the push rod adjustment.

[Zed Head]

Sorry about the confusion, I was thinking of the wrong valve, the NP valve versus the warning light switch. And I'm still not clear on how exactly the NP valve works. Can't tell for sure how the tandem MC isolates front from back either, I will have to dig in to it.

It sounds like you can have good brakes, but still not good enough for if one end fails. In 5th's case, the front brakes do most of the work anyway, so he should have had plenty of stopping power if the front and back are truly isolated. So it's not clear to me why he had essentially no stopping power when a rear wheel cylinder went out.

Do the brakes have to be perfectly bled to be safe, rather than just feel like they work fine?

Edit - just to add another thought. Both the brake warning switch and the NP valve have seals in them to isolate the fluids but to allow the transfer of some pressure, front to back and vice versa. Enough to move the switch for the brake check and to transfer some pressure for the NP valve. If the seals went bad, is it possible for the fluids to leak through? If so, you could have functioning brakes that are not isolated front to back. Bringing it up mainly for discussion, but also for safety.

Edited March 31, 2011 by Zed Head

[LeonV]

Sorry about the confusion, I was thinking of the wrong valve, the NP valve versus the warning light switch. And I'm still not clear on how exactly the NP valve works. Can't tell for sure how the tandem MC isolates front from back either, I will have to dig in to it.

All the proportioning valve does is limit pressure to the rear brakes under high demand. At a certain brake pressure, the valve restricts pressure as shown in the FSM. That prevents the rears from locking up before the fronts which would cause an unstable condition (ever pulled the e-brake when driving on snow?).

With that said, the proportioning valve is connected to the rear brake line, which is in no way connected to the fronts.

In a nutshell, the circuits are isolated in the MC since the primary and secondary pistons operate their own circuits. The primary piston is the one which is actuated by the brake pedal. The secondary piston is actuated by the primary piston as it builds pressure between the two. You press the pedal, the primary piston traps fluid and pressurizes it's circuit while simultaneously moving the secondary piston and pressurizing its own circuit.

If the primary circuit leaks, then the primary piston moves until it pushes against the secondary piston (no pressure builds between primary and secondary pistons), therefore actuating the front brakes in the Z.

If the secondary circuit leaks then the primary piston pushes the secondary piston to its stop by hydraulic pressure (no pressure between the piston and end of master cylinder) and then pressurizes the primary circuit, the rear brakes in the Z.

This book is very good if you're interested in studying the inner workings and all other aspects of brake systems.

It sounds like you can have good brakes, but still not good enough for if one end fails. In 5th's case, the front brakes do most of the work anyway, so he should have had plenty of stopping power if the front and back are truly isolated. So it's not clear to me why he had essentially no stopping power when a rear wheel cylinder went out.

Do the brakes have to be perfectly bled to be safe, rather than just feel like they work fine?

Bingo!

[Zed Head]

Thanks LeonV, that's about where my thinking was ending up as far as how the tandem system worked. Except that for my 1976, the primary would be the first piston (the one contacted by the rod from the brake pedal), which would be the front brakes (the first, big, reservoir) and vice versa.

So, essentially, if one system has air in it, the piston will run out of travel in its half of the bore before it can build up pressure to actuate the brakes, if the other system fails. That makes sense, and would explain the partial brake power - some pressure buildup but not enough. One good reason to shoot for a perfectly bled brake system, that I had not considered before.

It's easy to live with a little extra travel in the brake pedal, but I didn't realize it would defeat the safety factor.

[LeonV]

So, essentially, if one system has air in it, the piston will run out of travel in its half of the bore before it can build up pressure to actuate the brakes, if the other system fails. That makes sense, and would explain the partial brake power - some pressure buildup but not enough. One good reason to shoot for a perfectly bled brake system, that I had not considered before.

It's easy to live with a little extra travel in the brake pedal, but I didn't realize it would defeat the safety factor.

Precisely! Keeping the air out of your braking system is very important. This is a good lesson in keeping the brakes bled and when acquiring a used car, especially one that's approaching 40 years old, make sure to bleed the system. Previous owners tend to be terrible at basic maintenance, especially when you're like me and like to buy cheap Zs!

[tlorber]

Hey, while we're on the subject, should the brakes be bled with the motor off, or with the vacuum assist operating?