Brake fluid overflow

[inquirer]

After changing front pads and compressing the pistons a small amount of brake fluid overflowed and spilled out of the reservoir. Having changed the pads a couple of times (I change them often due to aggressive driving, often on mountain and downhill roads) and remembering that compressing the pistons wasn't causing an overflow during the 3 past pads changes, this time I didn't check the reservoir's level, but since I had changed the brake fluid a few months ago, this time the amount of the brake fluid was sufficient for causing an overflow.

The overflow happened with the reservoir's cap on.

I have 2 questions:

1. Does the cap have a design that allows overflow or did the overflow cause any damage to any parts of the system?
2. I know that brake fluids are corrosive. How likely is that they caused any damage to paint, cables or rubber parts (hoses and bushings) in the limited time (lets say 30 minutes max, probably less) it took me to clean the spill using water?

Thanks for your time reading my post, by the way!

 

[edyvw]

Was the reservoir closed or open? When compressing the piston, I always open the reservoir.
It will cause damage to the pain if there is paint below and you do not rinse it.

 

[inquirer]

Was the reservoir closed or open? When compressing the piston, I always open the reservoir.
It will cause damage to the pain if there is paint below and you do not rinse it.

As I wrote the reservoir was closed, but I didn't feel any resistance.
I rinsed the spill as soon as I installed the pads and the wheel and put the car back on the ground.

 

[cptbarkey]

1) Keep the cap off when doing a brake job. If it had a relief valve it would be obvious on the cap. I dont recall seeing one. You are probably ok.
2) It will definitely strip paint right off, but generally wont damage other stuff.

Next time keep an eye on the level and extract it. Better yet bleed the brakes and put new fluid in. Your brake performance will be much improved.

 

[eljefino]

There has to be a small relief in the reservoir cap, otherwise air would not get in as the pads wore. You were fine, cleaning up when you did.

 

[edyvw]

As I wrote the reservoir was closed, but I didn't feel any resistance.
I rinsed the spill as soon as I installed the pads and the wheel and put the car back on the ground.

Didn’t register with me. Always cap off, especially if Euro vehicle.

 

[tig1]

I always turkey baster the reservoir before compressing the caliper. You won't get overflow then.

 

[inquirer]

There has to be a small relief in the reservoir cap, otherwise air would not get in as the pads wore. You were fine, cleaning up when you did.

I guess there is, because I didn't feel any resistance. And I also guess there is because a vacuum would be created when the pads wore as you said, but it' isn't something obvious. There is a small groove running from the top of the cap towards the bottom. Maybe that's the intended way out for the fluid in case of an overflow.

How fast does the brake fluid corrodes things? I have heard that it would actually take days.

Didn’t register with me. Always cap off, especially if Euro vehicle.

I always turkey baster the reservoir before compressing the caliper. You won't get overflow then.

I always paid attention, but I never had to intervene. There was never a need to remove any amount of the brake fluid.
And this time I got complacent the overflow happened, because a fluid change had taken place a few months ago when the pads were worn to some point.

 

[nthach]

I’ve always popped the bleeder screw loose and pressed the pistons back home. Yes, you do need to bleed/flush the brakes when done but I rather risk air than pushing in moist and contaminated brake fluid into an ABS modulator.

 

[demarpaint]

I have a small syringe and remove some brake fluid when I do a brake job to avoid the possibility of spilling brake fluid when compressing the calipers.

 

[JohnnyG]

No air is supposed to get into the system, which would also allow moisture in. Usually, under the cap is a rubber membrane that is formed like an accordion. As the brake fluid is used (pads wear) that accordion pleated diaphragm looking thing expands so air doesn't get pulled in. HOWEVER, above that membrane, air will get in. So, you probably topped off the brake fluid at least once during the pad's usage, or someone else at a dealership or quicklube did it for you.
I'd make sure that the rubber expandable diaphragm doesn't have a hole in it, then you should be good to go.

 

[inquirer]

No air is supposed to get into the system, which would also allow moisture in. Usually, under the cap is a rubber membrane that is formed like an accordion. As the brake fluid is used (pads wear) that accordion pleated diaphragm looking thing expands so air doesn't get pulled in. HOWEVER, above that membrane, air will get in. So, you probably topped off the brake fluid at least once during the pad's usage, or someone else at a dealership or quicklube did it for you.
I'd make sure that the rubber expandable diaphragm doesn't have a hole in it, then you should be good to go.

Indeed there is an elastic membrane. As far as I remember the membrane is fine.
But if the cap is air tight how did the brake fluid overflowed. In that case it should be impossible to compress the pads and no overflow would have taken place, except if something broke.

 

[JohnnyG]

Probably pushed out under the membrane. I'd check it very closely for pinholes, but it could be designed to do that instead of splitting a plastic reservoir.
The old steel reservoirs had a kind of spring loaded hold down on the metal cap, probably for much the same reason. No way were you going to split a steel brake fluid reservoir, brake line or hose would be more likely.

 

[inquirer]

Probably pushed out under the membrane. I'd check it very closely for pinholes, but it could be designed to do that instead of splitting a plastic reservoir.
The old steel reservoirs had a kind of spring loaded hold down on the metal cap, probably for much the same reason. No way were you going to split a steel brake fluid reservoir, brake line or hose would be more likely.

There is a small groove running from the top of the cap towards the bottom. My guess is that this groove is there as a way for relief in case of overflow. It wouldn't make much sense anyway if the system has no safety mechanism in case the pressure in the tank increases. It would be weird if catastrophic failure was the only way of pressure relief.

 

[xmanjeff]

i wouldnt be a bad idea when compressing piston ,to run that fluid out of the bleeder , get that nasty stuff out of there

 

[inquirer]

i wouldnt be a bad idea when compressing piston ,to run that fluid out of the bleeder , get that nasty stuff out of there

In that case you have to refill it though. When the pads get worn there is a chance that the level of the fluid will be left lower than the acceptable minimum.

 

[JHZR2]

i wouldnt be a bad idea when compressing piston ,to run that fluid out of the bleeder , get that nasty stuff out of there

I was going to say… maybe I’ve done it wrong forever, but I always crack the bleeder, and let the fluid run out that way. I power bleed brakes on a regular schedule, and usually use a brake job as an excuse to do so too…

No way I’d want nasty fluid going back up into the piping and system. Get it out. Then flush it properly once the pads and caliper are back on!

 

[JHZR2]

In that case you have to refill it though. When the pads get worn there is a chance that the level of the fluid will be left lower than the acceptable minimum.

And??!? So what??

My process is always:
-open reservoir, remove nasty old fluid. Refill with fresh.
- perform brake job, do furthest location first
- open bleeder screw when pushing pistons back in. Close bleeder quickly with pressure on piston, to avoid air re-entry
- gravity bleed once caliper is installed
-pressure bleed all corners when done,starting with furthest

 

[KrisZ]

The cap is not air tight. It allows air in so that the membrane can get sucked in as the brake fluid drops with pad wear. Since the brake fluid flowed in reverse, it leaked past the membrane as it is not designed to keep hydraulic pressure.

 

[dumkid]

I only top up brake fluid when I put new pads on on my own car