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DOT 5 Fluid in my 240z - keep or flush?


Hardway

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Seeing that the brake fluid is a nice shade of deep purple I have confirmed with the previous owner of my '71 240z that it has DOT 5 fluid in the brake system. All of the brake components are either new or professionally restored but the pedal is very spongy. I plan on trying to bleed the system some more but I have also read about issues of using DOT 5 fluid such as how difficult it can be to get all the air out and that some brake components are not compatible with silicone based fluid. Should I flush all the DOT 5 out and put in some brand name DOT 3 or DOT 4? Looks like DOT 3 is always good and DOT 4 is good too as long as you stick to the flush interval of approx. 2 years. Any input is greatly appreciated.

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Using DOT 5 requires that all of the rubber parts of your brake system be resistant to silicone based fluids, to the best of my knowledge those parts arent even available for our cars. Given the uncertainty of what the PO did I would flush the entire system with DOT 4 brake fluid, not any type of petroleum product or a solvent. I always use DOT 4, its the same general chemistry as DOT 3 plus some additives so its completely compatible and has a higher temp rating, its also required in modern cars and motorcycles. Theres probably no downside to using DOT 3 though. Always use fluid from a brand new sealed container

A complete flush is certainly something you can do at home but it will take a few hours and lots of pedal pumping. A mechanic with a power bleeder could probably do the whole car in an hour or less so you might consider outsourcing the job.

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if you want to do it yourself - and keep up with it over time, invest in a set of speed-bleeders, it's a very well-spent $20.

these replace the nipples at the wheels and have a little spring/ball check valve in them so you just open them 1/4 turn, put on a piece of hose to a drain bottle and pump the pedal - the valve keeps air from going back in.

here's a link to some at summit racing - check that they will fit your car. i got a set from summit for my 78 280.

http://www.summitracing.com/search/product-line/dorman-brake-speed-bleeder-screws?SortBy=Default&SortOrder=Ascending&tw=speed%20bl&sw=Dorman%20Brake%20Speed%20Bleeder%20Screws

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About 8 years ago I rebuilt my dad's brake system in his black pearl 280z. I replaced everything- calipers, master cyl, wheel cylinders, etc. I knew that his car would sit a lot and his car has original paint. So, I decided to use silicone brake fluid. About 3 weeks ago I got the car running for him after sitting for probably 4 years. The brakes had not been bled for 8 years. The brake fluid looks brand new. I did bleed the system just to clear out any bubbles but everything that came out was like new purple fluid. So, the upside of silicone is that it can sit a long time without corroding the system. In my personal one car experience I didn't have any rubber compatibility problems. Silicone doesn't damage paint. The downside is you would not want to track a car with silicone; my understanding is the temp ratings are not the same as dot 4. The pedal also, as you mention, is softer. So, I do think silicone fluid has its place but you have to decide whether it is a match for your particular needs.

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I've been using DOT 5 for more than 30 years in all my 240Z's. The last brake master cylinder in the Blue 72 {I'm the original owner}, lasted more than 20 years. The car still has the original rear wheel cylinders. DOT 5 preserves the rubber seals - and it doesn't absorb moisture - so it all but prevents the usual corrosion you seen in the wheel cylinders and master cylinders.

DOT can be hard to bleed - but not so much if you don't get air bubbles in it to begin with - Don't pour DOT 5 out of the can and into an existing pool of DOT 5, or an empty reservoir. That is how you get air bubbles to begin with. - I use a "V" shaped piece of metal a little longer than a butter knife - and I poor the fluid out of the can directly onto that - and let it run into the Brake/Clutch master cylinder reservoirs.

Sound like the guy that owned the car - knew what he was doing. The only time you don't want to use DOT 5 - is for competition/racing. If you overheat DOT 5 - it becomes compressible and you get sudden brake fade.. If you are not melting the wheel bearing grease - you won't have any problems with the DOT 5, once you get the system bleed.

FWIW,

Carl B.

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Thank you for all the info and feedback guys! I worked on the Z today and decided to leave the DOT 5 in it. After I adjusted up the rears and bled everything with the help of my wife the pedal was nice and firm as it should be. I can definitely see the advantages of DOT 5 and since I am not tracking the car in any way it should be perfect for my needs. Turns out the real issue with the brakes was the rears had quite a bit of air in them and the driver side wheel cylinder needed 3 or more turns until it properly dragged on the drum. All in all a very productive day!

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Agree with Carl on this one.

I've used dot 5 in my Cobra replica from day one. Been seven years now, never had a brake issue, no corrosion,original fluid.

For cars that are driven sporadically I think it's great stuff. Don't have to worry about paint issues if you get a leak and it doesn't draw moisture like normal fluid.

Also use it in my 260 and the brakes work like normal, no soft pedal.

Used it in my clutch master as well.

Dean

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Jeff, like Carl B. and a few others here I have used Silicone Dot5 in cars and bikes that are not used on the track for years. It's great stuff.

If you do change back to Dot 3or4 make sure you flush the brake system with

Methyl Hydrate between types and then use compressed air to blow the system dry. Dot 3-4 are not compatible with Dot5.

Chris

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  • 4 weeks later...

How about switching from Dot 3 to Dot 5 - any special requirement or flush process necessary?

Also, since Dot 5 isn't hygroscopic, does that mean you can leave an open bottle on the shelf and it'll still be good to use in the future?

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  • 1 month later...

Brian,no. Don't leave it open!

I put DOT 5 in about 20+ years ago. Works fine. You must clean the system be fore using. Which means draining all the old out. I removed the flex hoses. And let lines and cylinders drain for several days. I have read you should flush with Mineral Spirits, but didn't and have not experience any problems. I guess there was minimum residual with my waiting as there would have been evaporation as well as draining of old fluid.

The added benefits for collectors is DOT5 does not damage paint.

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