So this is a strange one. I noticed that the brake fluid level at my master cylinder kept dropping, so I kept topping it off and watching for leaks. There is no evidence of leaks anywhere (and no puddles on the ground). Then I noticed that my ABS reservoir was filled to the brim, certainly above the MAX fill line.

What would cause fluid to transfer to the ABS reservoir? The ABS light is not on, and the system works like it should.

93 Accord EX. Thanks!

 

Also, is there a way to manually cycle the ABS solenoids?

 

The ABS system's reservoir is always filled above the max line, I'm not sure why but that is the way mine was. I would check your brake system more carefully for leaks, in drums if it has them, rubber lines, rusted hard lines underneath in that plastic sheathing for all the lines. How much is it losing? There is a way to cycle the solenoids. You will need to jump wires together on the harnesses. Here are the diagrams to determine which wires to jump, I looked but it hurt my head too much to actually try haha.

 

Thanks for the info. I've been checking for leaks and haven't found any.

I wonder if the slide piston in the ABS pack is leaking. I have no idea when the last time ABS actually kicked in. My theory: The ABS wasn't used for a long time and the brake fluid was old. The piston bore either has crud build up or is corroded. Once I activated the ABS, the piston moved over the crud and the now the seal has been broken.

Sound reasonable?

 

It does but the regular brake system and the ABS system are closed hydraulic systems and to a great extent, separate. If you have a broken seal in your ABS system, your gonna have a ABS light as well. If you have a given amount of fluid in the system then all that can/will happen is the act of applying mechanical force and the fluid acting to create the relative mechanical output so the ABS system robbing fluid doesn't make sense. I'm not too familiar with the hydraulic circuit but I do have the service manual and it even says that the two systems are separate at least when it comes to fluid replacement. Fluid may be being trapped in the ABS unit after the application of the regular brakes, like a one-way pressure valve may be sticking but I think your just noticing the high modulator level as I said. Also brake fluid is designed to support and maintain the condition of all the components it comes in contact with. Even if the ABS hadn't been activated for the last, say 5 years, it wouldn't cause any internal components to corrode. Like wheel cylinders for example, sure the o-rings fail eventually but a shot wheel cylinder rarely has internal corrosion.

 

Thanks for the information.

I agree, looking at the system schematics, it seems really unlikely that fluid could migrate between the systems. But I don't know what else it could be - fluid in my master cylinder keeps dropping, and the ABS reservoir keeps rising. I checked last night. (It was so high that it was actually overflowing in the ABS reservoir, so I siphoned some out.)

No ABS light and during the icy conditions this week I tested the ABS operation - works as expected.

 

Looks like I'm not the only one, but still unclear of the cause:

DIY for ABS Reservoir Overflow fix - The Acura Legend & Acura RL Forum

 

Sounds like a potential fix is to "exercise" the solenoids to clear up any debris that might be caught in there.

Now I just need to find another icy patch of road and go to town...will report back on the results.