Brake fluid level in reservoir question.

I did my first ABS brake flush today, it's a little different than standard brakes but not bad. The fluid was really dirty, hope it does not cause any problems down the road. My question is, when I pump the brake pedal twenty five times (engine off) to depressurize the accumulator , the fluid level is right on the money. Then when I start the engine and let it run a bit, then shut it down, the fluid level is about an inch below the full mark. Even if I do this a couple of times the result will be the same. Is this one inch gap for expansion, or do I have a problem. There are no codes and the brake and abs light go off when they are suppose to, after I start the engine.

Mike

If it is at the low mark after you allow the pump to run with the key in the on position and all the warning lights are out you are good.

If it is at the low mark after you allow the pump to run with the key in the on position and all the warning lights are out you are good.

I just reread the journal on the abs brake flush. Somehow I missed the bottom part that explains the low mark, sorry about that. I don't see a low mark on my reservoir, but I think I'm go to go. Thanks Dave.

Mike

On the side of the res. there are two triangles or arrow points, if you will, pointing at each other. The fluid level should be at the point where the two points meet. This is the level the fluid should be after the pedal has been pumped 25 times. When the key is turned to the on position, the pump will start and the level will drop because the fluid is being pumped back into the Accumulator. This is normal.

John F.

Edited September 15, 2014 by Machiner 55
changed "res." to "Accumulator" (see edit history)

If the fluid is at the full mark on the reservoir with the key off and the pedal pumped until the pedal is hard......then you have it filled to the correct level.

However... If it is dropping about 1 inch when the ignition is on, you better order a new accumulator.

Here is the logic and experience I have seen. As the accumulator looses its factory charge, the bladder deflects more when the pump gets to full pressure, this means more fluid is stored in the accumulator.

The more pressure lost in the accumulator, the more deflection and more fluid stored. From experience a new accumulator will drop the fluid level less than 1/2 inch. If you are getting a 1 inch drop your accumulator is on its last legs.

The caveat is we do not have a way of testing new accumulators, and many of us are suspicious that new ones may have been in inventory for years and they will loose some charge just setting on the shelf. If we had a known new accumulator, a bench test could be devised to measure the exact amount of fluid stored in a charged accumulator. With that knowledge we could establish pretty exact numbers for the fluid drop in the reservoir to rate the accumulator condition.

Here's the mark to fill to when the system is depressurized.

Supposed to drop 1/2 inch when system is fully pressurized as has been stated. If I'm checking the system when it is pressurized, I use the bottom of the black label as my guide. It's just about 1/2 inch square.

After looking at it again, key off , after I had the engine running, but on a cold engine, the accumulator should be pressurized , right? The fluid level is sitting between where it says full and do not overfill, just right on top of the "do not". I think that's about a 1/2 inch. Or are you saying I have to turn the key on to see if there is a further pressure drop.

After looking at it again, key off , after I had the engine running, but on a cold engine, the accumulator should be pressurized , right? The fluid level is sitting between where it says full and do not overfill, just right on top of the "do not". I think that's about a 1/2 inch. Or are you saying I have to turn the key on to see if there is a further pressure drop.

I thought I would add this to my last post. This morning I took the car on a test drive, on a hot engine the fluid level was sitting just above where it says "overfill" but it never goes below the little black label. I think what is happening, is when the fluid cools down it rises a little bit. I'll check it today to see if it does that. I'm thinking the brake system is okay, no lights , no codes, but I will keep an eye on future posts on accumulators. Thanks Guys.

Mike

... I think what is happening, is when the fluid cools down it rises a little bit. I'll check it today to see if it does that. ...

Here is what is actually happening... Let's assume the car has been sitting overnight and there is no pressure in the system. The brake fluid level should be at the top of the triangle next to the word FULL on the reservoir. When you turn the key to run the pump will start running. The pump pulls fluid from the reservoir and forces fluid into the accumulator which causes the fluid level in the reservoir to drop about 1/2". That is the normal level where the fluid should be as long as the key is in the run position. You don't want to add any fluid when the system is pressurized.

The fluid in the accumulator is separated from an inert gas in the accumulator by a rubber bladder (diaphragm). That gas was put in the accumulator at the factory under very high pressure. When you run the key off and let the car sit for several hours, or overnight, the pressurized gas in the accumulator slowly pushes the fluid out of the accumulator and back into the reservoir. That is the reason the level in the accumulator changes over a period of time when the car is turned off and that is why the fluid level in the reservoir returns to the FULL mark. Heat probably doesn't have much to do with the fluid level.

Edited September 14, 2014 by Ronnie (see edit history)

Ronnie, thank you for that explanation. Tomorrow, I'll recheck the fluid level, but I'm sure it will be okay. When I said it was about an inch low, I should have measured the drop to be more accurate. After measuring the black label, the depth measures about 11/16 of an inch, which is no where near an inch, I apologize for that. But it is still good to know the symptoms of a accumulator going bad and what to look for. Also when I filled the reservoir after the flush, I thought I had overfilled it, so I might have taken out a little too much. Plus there might have been a little residual air in the system after the flush. Anyway, I appreciate all the help.

Mike

To clarify what I posted earlier...

Self explanatory.

Level after 25 pedal pushes. Key OFF. Shows properly filled reservoir. Level should never be any higher under any circumstances.

Level with key ON and pump stopped. Do NOT add fluid. The pump has run and pushed fluid up into the Accumulator ball.

John F.

PS.,

McR. Sorry. Was working on this when you posted.

Edited September 15, 2014 by Machiner 55 (see edit history)

I feel cheated. My reservoir doesn't have the molded in down arrow. Wonder if this was a cut in change sometime.

Guess the last question to be answered is how long does it take for the fluid return to the fill line from the low level in photo 2 after the car is shut down, and what could we discern from deviations in that time? Weak accumulator, worn or leaking seals, gremlins are stomping on the brake pedal while we are sleeping? :confused:

My reservoir doesn't have the molded in down arrow.

Yah, I thought something like that might be going on so I removed the word "molded" from my earlier post (#4).

Would be a good thing to time how long it takes for the level to rise on it's own. Gotta work the next two days so I will be unable.

Need at least three different cars (more would surely be better) sampled to get an idea of a baseline.

John F.