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sorry to dredge this up again---weird brakes!


xray

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Hey all:

So when I bought my Z from the PO about 8 months ago, the brakes were not all that great. He said he replaced the MC. I added new pads, new flexible lines, bled the system multiple times and still have problems! Here's what I know:

1) The brakes engage without significant effort, but at the bottom of pedal travel. So, while the car stops and the brakes engage immediately, I cannot lock up the tires, and have no pedal travel... The pedal feels very "loose" until it finally engages.

2) If I pump the brakes with the car off, I can feel an improvement in the pedal resistance, and it almost--feels--normal. Until I turn on the engine when the pedal goes to the floor and we are back to Square 1.

3) Bad booster, right? Well, the engine generates 17 inches of Hg at the intake manifold, and I put 15 inches of Hg on the booster from the engine side, and it held the vacuum for five minutes. So there is no air leak in the hoses, I think the check valve is functioning, and the diaphrgam doesn't leak, so is the booster really bad?

4) Mucking around some today I noticed a small amount of oil on the boot just inside the firewall. Is it possible to have good vacuum on the diaphragm side, yet leak from the pedal side of the booster?

5) The guy's replacement MC has me wondering--it has two equal sized fluid reservoirs. As we know, on the 240s the front wheel reservoir is larger than the rear wheel reservoir, so perhaps this is the wrong MC? I remember when we dealt with Capt Mark's brake issue back in February, Kinfish and BillCapp stated to the effect that if the rod in the MC is too long, the pedal will have to move a lot to "make up slack" in order for the MC to compress the brake fluid and engage the brakes? Do you think the MC is the issue?

Any thoughts would be much appreciated.

Thanks

Steve

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It sounds like you have a M/C that is reversed. I bought a 15/16 M/C and it was reversed. I installed the lines and then I saw a post about the 70-71 Z's being different than the newer Z's. I was surprised that Datsun would make this change. Check it out to make certain the front cylinder is actually breaking the front system. I have not driven my car with the brakes lines reversed so I cannot tell you what happens. I think you will not have sufficient fluid movement to make you front brakes lock up. IMHO!

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This is just a best guess but have you checked that the reaction disc is in place in the booster.

It fits in behind the actuator rod inside the booster.

Sometimes when people change master cylinders, they will grab the rod and pull it out to check it, realise that it won't come out through the seal at the front and push it back in. This is when the rubber reaction disc falls into the bottom of the booster.

Worth checking as a lot of people don't know this disc is in there.

Hope this helps.

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To clarify:

It's a 1972. If the reaction disc falls out, doesn't that cause the booster to malfunction and not hold vacuum? The fact that it holds vacuum is the whole basis for my assumption that the booster is OK....

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I have the same EXACT problem and haven't found any solution but to disconnect vaccum from Booster. I have been told my many people that it is the booster, and I actually will have an other booster to test real soon. It could also be that the MC wasn't bench bled before it was put on, but I made sure that mine was and I still wound up with issues.

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If the reaction disc is removed the booster will still hold vacuum. It has no effect on vacuum what so ever.

The disc is rubber and is about 1/4" thick, if it is not in it's proper position it shortens the length of the actuation rod simply by the rod fitting 1/4" further into the booster.

The pedal travel will increase because the rod is now effectively 1/4" shorter.

Another side effect is the brakes when they do take up seem to grab because you have lost the cushion effect that the rubber disc provides between the bakerlite centre section on the diaphragm and the metal rod.

Hope this helps

Alan.

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Alan:

Thanks! It is entirely possible this disk fell out based on your description of what should happen, as it fairly matches what I experience in the car. How do I realign the disk? I am also unsure of the MC, thinking it may be for a different model (late 280) or may not have been bench bled prior to installation.

Steve

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Steve,

The easiest way to check if it is in there is to take it to someone who specialises in brakes and get them to check. They should also have a disc in stock and this would only take them 1/2 to 1 hour.

If you are confident to work on the brakes yourself, it's not a hard job.

Remove the master cylinder and alloy plate from the booster.

You will see the rod, seal retaining clip and seal.

Remove the retainer carefully leaving the seal in place for now.

Once you have the retainer out carefully pull the rod out untill it hits the back of the seal and use the rod to remove the seal at the same time as you remove the rod. There shouldn't be too much force required to do this.

If the disc is not stuck to the back of the rod , or you can't see it in the recess in the diaphragm, then you have found your problem.

If the disc is there then you have discounted that as the problem and we're back to square one.

When working on brakes you should never reuse old components, and only do the work if you are entirely confident you can do it properly. I can't stress this enough.

Alan.

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  • 1 month later...

Hey All:

Wanted to post a little update...

As you may (or may not) recall, I have been worried about the fact that my brakes don't engage until almost at the bottom of travel. Stopping, according to my car, was merely a suggestion, and had to be planned much far in advance. Impromptu braking? HA--a fantasy...

Thanks to Alan Pugh who brought up the idea of the reaction disk, not uncommonly falling out when a new MC is installed! The PO had a new MC installed (which I thought was a 15/16" based on the size of the reservoirs, probably mis-order for a 280zx...)

So I finally get some time off after working 68 days straight (including weekends). I crack open a beer and get to work. Drain the MC and pull off the steel brake lines from the master, remove it from the booster. Turns out it was a 7/8" MC with aftermarket fluid reservoirs...Go under the dash, pull off the four nuts holding the booster in (warning! The top left one is tough to get at, so be patient) and pry off the booster.

Thanks to ZVoiture and Alan Pugh who posted how they got to the reaction disk! After removing the diamond-shaped aluminum flange, I managed to save the retainer clip, and pulled off the seal and pushrod. Well, what do you know! NO reaction disk! I spent the next 20 minutes shaking the booster housing hearing it rattle around until it finally fell out. Put a little grease on that bad boy, stuck it back on the pushrod and back it went into the booster. Added some sealant to the pushrod seal and replaced the retainer clip. Bench bled the MC, reinstalled everything and bled all four wheels.

Now the pedal feels tight! Hopefully it still holds vacuum......

Thanks for the help--could not have done it without you.

Steve

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Attaboy to Alan Pugh's suggestion. It is great to hear that some readers follow troubleshooting suggestions precisely and when they do they find the root cause of the problem. Without the reaction disc in place you were consuming pedal travel just to make contact with the master cylinder piston, assuming the rod was adjusted properly with disc in place.

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I have printed out your posts for my records. I am not quite ready to do the brakes. I just rebuilt the 5-speed and the engine block is finally done at the engine shop. I will pick it up today. Once I get the engine dialed in and installed, I will start on the brakes. Your info is invaluable. Where were we 25 years ago? We were finding this info out all by ourselves. How things have changed!

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  • 3 weeks later...

How tight IS the brake pedal now that you've fixed the problem? My brake pedal has some travel and its hard to lock up my wheels, but it's not horrible. Could it be that my drum brakes need to be adjusted, or is it possible that I have the same problem? (a couple years ago I had a new M/C put in, so I dunno if they lost the reaction disk, but my braking isn't that bad, just some pedal travel.)

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