Hello 2024 Zeason and fellow enthusiasts.

I just woke up the 1985 Z from its winter sleep and noticed that the "BRAKE" light didn't go off when I released the hand brake. Figured that the switch for the brake was "stuck" until I pulled it out of the garage and saw that the cardboard that was placed under the car was stained with oil. The brake reservoir was down, below the minimum level mark, so I topped it off with DOT 3. Oil was running down the boxed frame rail from the master brake cylinder onto the deck. Had good height to the brake pedal and everything seemed to be working normally so I slowly rolled down the street, stopped several times with the brakes stopping Z car without any issues. The dashboard "BRAKE" light was no longer glowing. I pulled the Z back into the garage and moved the refreshed floor cardboard under the master brake cylinder. The reservoir has remained full and the cardboard is still clean and dry after three days.

Did the seal shrink or dislodge over the winter and then expanded or resealed itself when the brake power booster applied pressure? There are after market replacement master cylinders available. Should I pay the $350 - $550 labor cost to have one installed since it's a job I'm not able to do myself anymore? Or, do I just keep a close eye on the reservoir level, pedal height/feel and the braking performance? I really don't want to wind up playing bumper-cars with other drivers on the road.

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1985 300ZX- GLL Dark Pewter Metallic - Survivor

I would keep an eye on it Howie. The seals can leak if not "exercised" for long periods of time, so I would recommend running the car at least up and down the driveway every couple of weeks and use the brakes. If the fluid level continues to drop and shows up again on the carboard, I would service or replace the master and check the slave while you are at it. I also change out my brake fluid every couple of years.

Cheers!

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John Kish
1971 240Z - original owner

Hi John. Thanks for the advice. It hasn't leaked after exerciZing the brake system and a parkway run, including panic stops when no other vehicles were around. Performance was as-it-should-be. I removed the brake fluid from the reservoir and replaced it with fresh. All appears to be back to dry and leak free. Have a great summer.

Howie

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1985 300ZX- GLL Dark Pewter Metallic - Survivor

I would pull the master loose from the booster to make sure the back seal isn't leaking in to the booster.

Howie,
Forget about what I said about the slave cylinder as I was thinking of the clutch for a moment. A leaky master can leak into the booster and also your driver's side floor as described by our members previous post - Thank You. My concern is when was the last time you bled all the brake fluid from each wheel in addition to the master cylinder? Brake fluid is hydroscopic (absorbs moisture) so it is important to change the fluid by completely bleeding all wheels and master periodically. Cars that are stored for a long period of time can, and will, have seals deteriorate over time at the wheel cylinders (drum brakes), and calipers (disk brakes) as well as the seals in the master cylinder. Moisture build up in the fluid can also damage and corrode those metal parts containing the brake fluid. Same goes for clutch master and slave cylinders. That's why it is so important to exercise these systems by driving and car every few weeks if only up and down the driveway and to change the fluid every couple of years.

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John Kish
1971 240Z - original owner

Thank you Ktl240 3.0 and John for your input. If I back off the two nuts that are holding the brake master cylinder to the booster do I have to remove the brake lines or will I know if the back seal is leaking as soon as I "crack it loose" from the booster?
Thanks.
Howie

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1985 300ZX- GLL Dark Pewter Metallic - Survivor

Howie, it may be worth a try.

I'm not familiar with the '85 master/vacuum booster design. In my '71 it is a pretty easy process of removing and checking for leaks and even replacing the master cylinder. The pain for me is having to bleed the master and all four corners. My wife isn't a big fan of me yelling up from under the car...."up.....down...up...down" directing her to operate the brake petal.

Since brakes (and tires) are top maintenance safety priorities you might want to just "bite the bullet" and bring your precious Z to a reputable mechanic.

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John Kish
1971 240Z - original owner

Brakes are important. Agree with the points about making sure you change fluid just make sure you keep the system topped up with new clean fluid otherwise you will need to bench bleed the master. If you losing fluid you really need to find the source. Weird things can happen including brake lines ballooning or getting blocked with crud. Hear you on getting someone to help bleed brakes. A bleeder container with a one way valve if you get a good seal on the fitting can save a lot of aggregation in (from helpers:))

I have used this one. Just be quick to close fitting.

https://www.amazon.com/dp/B00F9XHVKA/re ... 29d5&psc=1

Changing the master cylinder is an easy uncomplicated job, the majority of the time. Bleeding the system and finding someone to "pump, pump, pump, HOLD IT DOWN!" about, what used to feel like 100 times while lying on my back and trying to keep the bleed tube in the receptacle every time I would open and close the bleeder was/is the pain in the butt part.

The one person brake bleeder sounds like a good idea but doesn't it require a suction pump? The reservoir level has not gone down at all. I presume that if it were leaking into the booster that the fluid level would be down. Thanks for your inputs. I greatly appreciate it.

Howie

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1985 300ZX- GLL Dark Pewter Metallic - Survivor

No vacuum needed. Just make sure hose on fitting is tight and work quickly. As some one suggested you should replace fluid. One way is to use a baster to take out fluid and then replace with fresh fluid and then bleed a few times to make sure fluid coming out is clean. Top up frequently. Also do not mix silicon and regular brake fluid.

Bruceg thanks for the info. But why can't Dot 3 brake fluid be mixed with Dot 3 silicone brake fluid? I would think that it would be similar to mixing regular motor oil with synthetic motor oil.

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1985 300ZX- GLL Dark Pewter Metallic - Survivor

Here's what I found after Googling: regular,heavy duty, synthetic and silicone brake fluids. Compatibilities and differences, pros and cons.

Can you mix synthetic brake fluid with regular brake fluid?

Brake fluids can be either mineral or synthetic. You can mix them without issues, provided they are rated DOT 3, DOT 4, or DOT 5.1. Both regular and synthetic brake fluids are miscible.

So, if you are asking, can you mix synthetic DOT 3 brake fluid with regular DOT brake fluid? Yes, you can mix them without any issues.

However, you can not mix synthetic and regular brake fluid if any of them is rated DOT 5. DOT 5 brake fluids are silicone-based and incompatible with other fluids with different DOT ratings.

Silicone based brake fluids(Dot 5.1?) appear to be used in most/all race cars and should not (never, never ever) be put into Dot 3 or Dot 4 brake systems.

Howie

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1985 300ZX- GLL Dark Pewter Metallic - Survivor

You guyZ are a wealth of information

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1970 240Z

Believe DOT 5 is Silicon but DOT 5.1 is not (Glycol). Make sure to check labeling when you buy.

Also do not believe folks use Silicon in race applications much. Believe it can have a spongier pedal feel but does not remove paint if it spills though.