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Where is the brake fluid going?

1999 Honda Civic. Have had to add brake fluid 2 or 3 times in a month. No leaks are evident at master cylinder, brake lines, calipers, at rear wheel cylinders (I took off the drums). Where can the fluid be going? Can it exit the master cylinder and pool up in the power steering booster? Other places?

A small leak into the booster could siphon off brake fluid. Not sure if that is the case with your car.

Ususally brake fluid is lost somewhere from the lines without showing puddles on the ground. You need to take the car to a good shop (not a tire shop) since this is potentially a very dangerous defect.

Take it to a brake specialist. You don 't want a slow leak to become suddenly catostrophic while driving 60mph down a hill.

Loosen the nuts holding the master cylinder to the power booster. If fluid drips out the master cylinder is bad. Replace it ASAP. The master cylinder will totally fail soon and the fluid will eventually spill over into the booster and deteriorate the diaphram. In fact, if there is considerable fluid spilling out it might be worthwhile to buy a “loaded booster” which is a booster and master cylinder assembly often priced at a considerable savings.

Sooner is much cheaper than later.

I wholeheartedly agree with Rod’s post, but wanted to add that if the rear wheel cylinders had crud built up on them they probably have failing seals as well. Often a small leaks in the cylinders will show up as a crud buildup rather than a fluid residue path.

Look on the insie of the firewall in the drivers compartment, if the carpet or mat is wet with brake fluid it could be leaking out of the rear of the master cylinder.

you mean brake booster not steering booster

I removed the master cylinder from the booster (yes, brake, not power steering, booster) and found no fluid behind the MC.

I had it checked at a shop on Saturday. They took off drums, looked closely at calipers, pressurized the system and found nothing wrong. I guess what thay say is true - that brake fluid level will go down as pads, etc. wear. First time in almost 50 years of driving that it happened to this extent.

Thanks to all who joined the discussion.

Adding 2-3 times a month is not pad wear.

The master cylinder should be at the full line when the pads are new. The fluid level will drop as the pads wear. You may get a brake light when the pads are warn out. Putting new pads in will fill the reservoir to the full line again. I have never “topped” off brake fluid. It a closed system and if you have lost fluid you have a problem. Especially if it is 2-3 times a month.

As knfenimore described, brake fluid level in the master cylinder will drop as the pads wear.

But, as Texases said, having to add fluid 2-3 times a month is NOT pad wear. It’s seeping out somewhere. I suggest you make sure your parking brake is working until this gets resolved, just in case you need to use it as an emergency brake. And leave plenty of space between you and the car in front if you, which is always a good idea anyway.

I have topped off my own reservoir to stop the light from going on & off, as it will when the level gets low, but knowing that it’s time to check the pads. Topping off the reservoir is not a “fix”. It’s designed to let you know you need your brakes checked.

Update: I still had to add fluid a couple times in the last month, after having the brakes checked at a shop I trusted. This AM I investigated under the car, and found evidence of a leak seeping out from the plastic cover that’s to the left of the muffler. Most likely it’s brake fluid, leaking from a pipe or fitting up there.

It’s not a job I want tackle right now on my back in an unheated garage in Duluth, so I’ll take it somewhere for repair. This does not seem like a very expensive thing. I’m disappointed with the shop that failed to diagnose this a month ago, after I had added close to a pint, but maybe it took a long time, and more fluid, for it to show up.


Thanks for the update

And please let us know what the leak turns out to be

One of the brake lines was leaking. I had them replace both, all the way from the firewall to each rear brake. Problem solved for $360 - the most expensive mechanical repair I’ve had in 152,000 miles and almost 15 years with this car.