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Old 10-24-2001, 01:30 AM
Posts: n/a
Question A question for Benzmac or anyone who may have an answer

Benzmac, in my brake pad/rotor change posting, I had a big concern about how the brake fluid soaked my hood pad. I have a question that is based on your reply:

I have seen this over and over. When you do a brake job, you have to take off the lid and put a rag over it so the rising fluid will not spray out of the little hole. The little hole in the top of the lid is for inlet air and is small, when pressurized by an idiot, it will spray out like a spray bottle.

Suppose the lid was left on tightly, and the only way the fluid got out was through the tiny inlet hole by means of high pressure. Just how powerful is this pressure that caused the fluid to shoot out? Should I be concern that it may have caused leaks or potential problems in the brake system? The hole is rather small, and I would think it insignificant as a means to relieve pressure.

But then again, the stain on the hood pad is quite big, about 8-9" across and 6" wide above the resevoir. It's hard to imagine that such a tiny hole could have sprayed the area. But your experience can confirm this. My other theory is that they were aware of rising fluid during pad changes and unscrewed the resevoir lid. However, they did not put a rag to absorb the fluid but instead closed my hood down with the idea that my pad would absorb it! That kind of explains why the lid had no messy fluid on it. That angers me deeply--how could they!

I don't know which is more comforting, the pressure hole or the pad rag. Which is more likely the case? And can I tell by the fluid splashes on and around the resevoir? Which situation is most likely to cause heavier spills around the resevoir area?

One more question, and I won't bug you again. What is the purpose of the inlet hole on the resevoir lid? I had sprayed some brake fluid cleaner on the stain (with a cloth over the resevoir), and I then closed the hood. Would the cleaner fumes have any ill effects on the brake fluid inside the resevoir because of that tiny opening? Thanks a bunch. It's kind of sad that I paid so much for a job and not have any questions answered truthfully, if I even got them answered.

Last edited by cossie; 10-24-2001 at 01:37 AM.
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Old 10-24-2001, 07:23 AM
engatwork's Avatar
Join Date: May 2000
Location: Soperton, Ga. USA
Posts: 11,877
The hole is a vent that allows air in. If you will notice as the pads wear the level in the reservoir falls. The fluid is having to fill the "larger" space behind the pad in the caliper cylinder so something has to take the place of the fluid that is now missing from the reservoir. I make it a habit to fill the reservoir to the "full" line after installing new pads and when the level gets to the min line I pull the wheels off and check the condition of the pads. Sometimes - based on the vehicle (the Honda comes to mind) the pads are ready to change when the level gets to min.
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Old 10-24-2001, 08:16 AM
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,784
It does not take much pressure to squirt out the vent hole. [like a water pistol]
Another reason to open the bleeder when pushing the pistons back in when changing pads.
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Old 10-24-2001, 04:38 PM
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smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,115
Don't worry about the pressure of pushing a piston into the caliper stressing the brake system. Normal application of the brakes puts a lot more pressure on the system.

Brake fluid has a tremendous ability and willingness to flow (I don't remember the technical term for this characteristic). That's why it's also a good penetrating fluid for stuck nuts and bolts. It is very possible that a concentrated stream from the vent hole shot into the hood pad and spread over time. I'll bet it's still spreading now.

If they had the cap off without a rag, fluid would have spilled over the top of the reservoir and dripped down the master cylinder. If they jammed the piston into the caliper, there might have been enough force into the reservoir to throw a blob of fluid onto the pad. But definitely more around the master cylinder than in the pad.

91 300SE
81 300SD
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Old 10-24-2001, 05:04 PM
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You should always open the bleed screws before pushing the brake pistons back,otherwise used fluid from the calipers is pushed back up into the master cylinder,not a good idea
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Old 10-24-2001, 05:21 PM
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Join Date: Oct 1999
Posts: 1,562
this is all well founded experienced advice from the forum

...the brake system (master cylinder, lines, etc.) should be fine (it just experienced stress/pressure expending the extra brake fluid= cap on or cap off-...and experiences probably even more pressure with road duty by comparison)

if you are concerned about the spotted hood pad, request the shop to put a new one it

more of concern would be if the brake fluid happened to fall on any painted surface (inside the hood or outside on the fenders!)......result: no more paint- like magic

good luck and hope everything turns out
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Old 10-24-2001, 06:30 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
I didn't reply to the original post as I presumed my opinion might not have much weight me being a shop owner.

Having sold brake service for nearly thirty years, this issue is probably the least mistake I have seen. I doubt that it has happened in our shop in at least ten years as the bleeder technique solves the problem.

As stated above the only damage is the stain on the hood pad. It might be beneficial to rinse the pad with water. I would spray it with a windex bottle and then flood the area it drains to. A hair dyier should dry it out. Leaving the brake fluid in the hood pad will probably destroy the painted undersurface of the hood (above the pad).

I would catagorize this as a minor non-safety related indescretion. I think the hood pad issue can be dealt with without replacement but the shop probably would be willing to replace it if that brought satisfaction. The issue to me is minor carelessness or working under too much time pressure. My advise is accept the incident as an accident. let them bring you satisfaction and if they do decent otherwise keep using them. If this works out you have the basis for a strong relationship. I believe having a strong relationship with your technician or shop brings great value.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 10-24-2001, 08:23 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
Steve brings many good points and views to the table. IF you trust these guys, wash it and let it go. Let them know that it was a mistake and you will let it slide...

If you don't trust them, why use them in the first place?
Get a new pad installed and find a new shop/tech.
Donnie Drummonds
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