Fluid leak when swapping Brake pads

Hi Experts,

I was swapping my front pads yesterday and while the front was jacked up I noticed a puddle of fluid on the floor on the driver side where the jacking points are and also a puddle of fluid in front of the driver side rear wheel. I looked under and saw that it was leaking from the under panel but couldn't track where the actual leak was from. The fluid was pale yellow and had a water-oil consistency but not water for sure. I felt like it was Brake fluid. The leak disappeared after I swapped the brake pads and the car was back level on ground. I took the car for a short drive at slow speeds and it appeared to be braking fine. Not sure where the leak was from. Any one experience this? Am I ok to drive it or should I have it checked it although I will have a hard time replicating it I think.

P.S: There was no leak from the caliper or anything when I was swapping out the pads.

M'EM wrote

Hi Experts,

I was swapping my front pads yesterday and while the front was jacked up I noticed a puddle of fluid on the floor on the driver side where the jacking points are and also a puddle of fluid in front of the driver side rear wheel. I looked under and saw that it was leaking from the under panel but couldn't track where the actual leak was from. The fluid was pale yellow and had a water-oil consistency but not water for sure. I felt like it was Brake fluid. The leak disappeared after I swapped the brake pads and the car was back level on ground. I took the car for a short drive at slow speeds and it appeared to be braking fine. Not sure where the leak was from. Any one experience this? Am I ok to drive it or should I have it checked it although I will have a hard time replicating it I think.

P.S: There was no leak from the caliper or anything when I was swapping out the pads.

Did you loosen the brake fluid reservoir cap while changing the pads and some fluid sloshed out when you jacked the car up? I would check the reservoir and see if it's low or empty.

Maybe pushed some fluid out of the reservoir while compressing the pistons to fit the new pads???

dparm wrote

Did you loosen the brake fluid reservoir cap while changing the pads and some fluid sloshed out when you jacked the car up? I would check the reservoir and see if it's low or empty.

McSwine wrote

Maybe pushed some fluid out of the reservoir while compressing the pistons to fit the new pads???

This is likely what happened, pushing the pistons back brought the fluid back into the reservoir.

Make sure the excess brake fluid didn't touch the pressure sensor (#12), as they are very sensitive to moisture and can short - resulting in limp mode during driving:

Thanks for your replies everyone. To answer some of your questions no I did not open the reservoir cap and I just checked the reservoir, it is full to the top.

So you guys mean when I compressed the cylinder fluid got pushed to the reservoir and it leaked from there? Is that ok? Can I drive the car now or should I bleed the fluid to make sure there are no air bubbles?

Anyone? I just want to be sure that its ok to drive the car. I may sound silly but if I pushed fluid out of the reservoir does it mean I could have introduced air in the lines? Can someone chime in please..

M'EM wrote

Anyone? I just want to be sure that its ok to drive the car. I may sound silly but if I pushed fluid out of the reservoir does it mean I could have introduced air in the lines? Can someone chime in please..

M'EM wrote

Thanks for your replies everyone. To answer some of your questions no I did not open the reservoir cap and I just checked the reservoir, it is full to the top.

So you guys mean when I compressed the cylinder fluid got pushed to the reservoir and it leaked from there? Is that ok? Can I drive the car now or should I bleed the fluid to make sure there are no air bubbles?

You're fine. No need to bleed. Just clean up the brake fluid with some brake cleaner and a rag, and move on.

Remember, if you check your brake fluid and it's low, but you don't find a leak, DO NOT ADD FLUID.

it means your brake pads are worn out and you probably need to change them. If you add fluid to the reservoir and then compress the caliper piston, it will force the fluid out of the reservoir and all over the engine bay/ lower tray.

Adenau wrote

You're fine. No need to bleed. Just clean up the brake fluid with some brake cleaner and a rag, and move on.

+1, it happens. Paint work is the most likely victim during such spillage, as the brake fluid is corrosive for paint.

If you are not sure if the brake fluid has been topped up since all four pads were changed last, then it is best to extract some brake fluid from the reservoir before renewing pads.

M'EM wrote

Anyone? I just want to be sure that its ok to drive the car. I may sound silly but if I pushed fluid out of the reservoir does it mean I could have introduced air in the lines? Can someone chime in please..

If you never either opened or loosened the cap, that should not be the source of the fluid spillage, and your first job should be to find the leak. If you did in fact loosen the reservoir cap it is likely that when you pushed the pistons out to make room for the thicker brake pads brake fluid forced its way out of the system through the threads on the cap. The brake system is a closed loop, meaning that unless there is a leak or you have added or replaced the brake fluid there should not be air in the lines.

Cap is culprit for sure. Must open.

DSilk wrote

If you never either opened or loosened the cap, that should not be the source of the fluid spillage, and your first job should be to find the leak. If you did in fact loosen the reservoir cap it is likely that when you pushed the pistons out to make room for the thicker brake pads brake fluid forced its way out of the system through the threads on the cap. The brake system is a closed loop, meaning that unless there is a leak or you have added or replaced the brake fluid there should not be air in the lines.

Thanks for pitching in. I can confirm that I haven't loosen the Reservoir cap. In fact I dint even open the hood when I did the pad change. I m not sure if it was loose to begin with. How do I find the leak if its not leakin anymore. I am more worried about any air that could have been introduced because if it's a closed loop with brake fluid filled in from start to finish and then some of the fluid is removed (via the leak) then what happens to that void? Is it not logical that there is an empty space in the loop....or do I not know what I am talking about??

Candide13 wrote

+1, it happens. Paint work is the most likely victim during such spillage, as the brake fluid is corrosive for paint.

If you are not sure if the brake fluid has been topped up since all four pads were changed last, then it is best to extract some brake fluid from the reservoir before renewing pads.

Yes..I would think so the cap was loose. I cleaned up under the car where the fluid was leaking from but I cant reach the other areas where fluid could have passed through. I just hope it dint mess up anything.

Reservoir cap is vented, if you added fluid in between pad changes - it can leak out through the cap when pushing back the pistons.

M'EM wrote

Yes..I would think so the cap was loose. I cleaned up under the car where the fluid was leaking from but I cant reach the other areas where fluid could have passed through. I just hope it dint mess up anything.

Sounds good. BTW, the cap venting design allows overflow spilling without the cap being loose, this is a safety feature to avoid pressure build up in the reservoir.

Candide13 wrote

M'EM wrote

Yes..I would think so the cap was loose. I cleaned up under the car where the fluid was leaking from but I cant reach the other areas where fluid could have passed through. I just hope it dint mess up anything.

Sounds good. BTW, the cap venting design allows overflow spilling without the cap being loose, this is a safety feature to avoid pressure build up in the reservoir.

I had the same leak as OP is describing after doing a complete rotors and pads replacement job. The venting would explain why it has been OK for the past 5k miles. I never did find that "leak", and I would've expected air in the system by now if it was a real leak.

Candide13 wrote

Sounds good. BTW, the cap venting design allows overflow spilling without the cap being loose, this is a safety feature to avoid pressure build up in the reservoir.

Thanks for the response. That's the first time I am hearing about the venting design on the reservoir cap. Good to know. So I guess I am all good.

TWiTCHY wrote

I had the same leak as OP is describing after doing a complete rotors and pads replacement job. The venting would explain why it has been OK for the past 5k miles. I never did find that "leak", and I would've expected air in the system by now if it was a real leak.

Did you happen to clean all the areas where fluid passed...any issues as far as paint us concerned?

tom @ eas wrote  

dparm wrote

Did you loosen the brake fluid reservoir cap while changing the pads and some fluid sloshed out when you jacked the car up? I would check the reservoir and see if it's low or empty.

McSwine wrote

Maybe pushed some fluid out of the reservoir while compressing the pistons to fit the new pads???

This is likely what happened, pushing the pistons back brought the fluid back into the reservoir.

Make sure the excess brake fluid didn't touch the pressure sensor (#12), as they are very sensitive to moisture and can short - resulting in limp mode during driving:

This. Had the CEL/limp mode after a BBK install from a damaged pressure sensor due to brake fluid overfill. Hopefully it doesn't happen to you.

M'EM wrote

Did you happen to clean all the areas where fluid passed...any issues as far as paint us concerned?

I saw the leak and read around on other forums that said it was normal when compressing the piston, so I didn't think to go looking for it, nor did I clean anything as the leak came from the underside of the car and did not make contact with my paint.

This is with AP Racing brakes, but while changing pads at the track a couple of times there was a tiny bit of fluid leaking past the pistons after pushing them back in. Is completely normal. Also, with respect to prior comments, you can disconnect the brake reservoir sensor and clean it. Pushing back pistons does cause the reservoir levels to rise up too high and can get on the sensor.

Saandiaago wrote

This. Had the CEL/limp mode after a BBK install from a damaged pressure sensor due to brake fluid overfill. Hopefully it doesn't happen to you.

Another victim here. OP better knock on wood. Mine was fine for about 10k miles then decided to finally short out.

If you ever need a reference, there's some good info in this thread:
http://www.m3post.com/forums/showthread.php?t=1338052