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A bit low on brake fluid...can I just add?

2009 Lexus ES350 80K miles

While replacing the battery, I noticed the brake fluid was at the MIN line. Since the fluid is well over a couple years old, I know the proper way is to dump and replace with new. This is something I would like to wait until my next oil change at the shop. In the meantime, do you see any harm of me adding new fluid just to get it up to the middle of MIN and MAX line?

There’s no harm…in fact you should. Make sure you use the correct fluid…I think it’s DOT-3.

No harm whatsoever.
However, you also need to have your brakes checked. The relationship between the reservoir capacity and the system capacity is such that low fluid in the reservoir means that the fluid has traveled to and been retained in the calipers because most of the pad material has worn off.

Put simply, a low brake fluid reservoir means you need to get your brakes checked.

Exactly what I was going to suggest …likely worn brake pads.

As has been explained above, normal brake pad wear will cause a drop in the fluid level. By the time the pads are worn to being near needing replacement, the fluid level will be just about at the minimum. It’s an indicator that the brakes are worn and need inspection/replacement as needed.

It’s also the reason we never top off brake fluid as part of an oil change service.

That pedal drop from pad wear can result in a brake warning light due to low fluid level.

Make sure after you fill the reservoir that you push the rubber diaphragm’s “buttons” (located in the lid) back up and into place before putting the lid back on. There could be one or two of these buttons. The buttons are drawn down as the brake fluid level drops.

The reason I don’t top off is b/c when the fluid is low , as mentioned above, something in the brake system usually needs to be replaced. All the pads/disks/shoes/drums dimensions need to be checked. The problem with topping the fluid off in the meantime is that when you replace the worn part, the fluid will then overfill the bottle and create a big mess. You can of course suction it out beforehand, but besides being just another job you’ve given yourself needlessly, you also risk contamination from the suction device.

i worked on a friends car 5 yrs ago. did a brake job. than i bought the car this spring. had to add some brake fluid last week. maybe the brake pads are getting thin? is it serendipity that i had to add fluid now? i never checked it 5 yrs ago.

I would add very little, just to bring it up above the minimum line. Be sure and use new fliud from a fresh container. Low reading without a leak indicates your pads are worn. Have pad s checked.

“Be sure and use new fliud from a fresh container.”

I cannot argue with this ‘conventional wisdom’ as a safe approach. But has anybody seen any real data that says, say, a 1 qt bottle of Dot 3 brake fluid, used once, left half full, and tightly recapped, will have a dangerous level of moisture in it after a year on the shelf?

It depends on how good the seal is on the container. When the cap is opened on a new container and fluid is poured out, ambient air comes in. But I wouldn’t guess that would result in much water getting into the fluid. The way water could get into the fluid is if ambient air was constantly circulating into the can, like from thermal siphoning. But that could only occur if the cap seal wasn’t air tight. It seems like if the seal is air tight, there wouldn’t be much chance for water to get in used brake fluid.

After a year on the shelf ? No. And you maybe right for advice for a car nerd like you and I. But this is an OPer in general , who may have any on hand and more often then not, could have a buddy who has. Loose capped rusted metal container sitting in a shed out back for who knows how many years of brake fluid which if you looked in any garage that had used brake fluid is what I envisioned. This maybe wrongfully so but I would like to err on the official CYA position.

I tend to buy new brake fluid for the brakes, and “demote” any surplus fluid to hydraulic clutch service. Seems the “worst case scenario” of an INOP clutch is a bit more liveable than the WCS of an INOP braking system.

The “6 months on the shelf” brake fluid is why they sell those little containers of fluid so you can use it and recycle the rest. It is glycol so you can recycle it with used coolant. They are also cheap so you don’t feel so bad tossing it with half a container left (or use it in clutches…)

I write the month and year on the top of the container so I know when it was opened. More than 6 months and it gets tossed.

Dollar Tree stores have 8 ounce plastic bottles of DOT 3 brake fluid for a buck. Very handy if you don’t need much.