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Has anyone ever seen this before brake master cylinder?

Back in July I did a front a front brake job on my truck everything went smoothly & had good brake’s. 4 week’s later my brake light came on still had good brake’s found the master cylinder was low added half of a 12 oz.can to bring it up to level thought there must have been a slight bit of air that did not bleed out so thought no more about it. 4 week’s later same thing checked a lot closer & found a slight leak in master cylindrer changed it out for a new one. . Since then I have been noticing every time I open the hood the level has been rising now it is about an inch fuller than it was after the new master I have never seen this before how would you explain this? 1990 F 150

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When you changed it out for a new one, did you bleed the whole system, or did you just bench bleed the master cylinder and then fill it after install?

Does the level drop back down again after the truck’s been sitting for awhile?

Bench bled the master & all 4 wheel’s & no level does not drop it seem’s as if something weird is going to happen it happen’s to me.

You can clean the reservoir, and with the engine running have someone pump the brake pedal a few times and then depress it hard and hold it and mark the fluid level. Then release the pedal and immediately mark the level. Then after an hour mark the level. If there are 3 distinct levels marked occasionally check the fluid level for a consistent pattern to the level and if there is I would guess that the proportioning valve is slow to fully bleed down.


I will give that a try it took 9 month’s to get where it is now so I am not really worried about it I don’t really look that close at every time I open the hood as it is like seeing a kid grow if you are around all the time you don’t notice the growth but if you only see them 3 or 4 time’s a year you can see the difference.

Well something has to be pushing it up. Law of gravity and all that. hmmm … If you have rear drum brakes my guess is the shoes were adjusted a little on the loose side and over time they’ve self-adjusted themselves, which pushed the fluid in the wheel cylinders back toward the MC. Something like this could happen with a disc caliper piston too I suppose. There’s a seal in the caliper that has an elastic force that tends to keep the pad just barely touching the disc w/the brakes not applied. If that elastic force changed and the pad was not longer quite upside the disc, that would tend to push the fluid back toward the MC. I can think of some air bubble theories, but they seem very unlikely.

Or maybe somebody is messing w/you and pouring a little brake fluid in the MC when you aren’t looking. I think Tom and Ray mentioned a prank like this, but I think it involved gasoline, not brake fluid …


If you pedal is still firm, George San Jose’s theory about the rear brake adjustment it the only thing that makes sense to me. If the pedal is getting soft, then you are somehow getting air in the system without losing fluid.

A WAG just occurred to me. Is there a vacuum line to your transmission and could tranny fluid be getting into your brake system through you brake booster?

Rear brake’s are adjusted with slight drag when turned by hand the pedal stay;s hard I can’t see how any air is getting in the system it is a manual transmission so no vacuum line.

Are you parking in the same spot facing in the same direction when you observe the level? If so, you hve me stumped/

Same level spot or as level as you can get on a get on a gravel driveway where I check oil & tire’s every week or so. My mechanic who doe;s any work that I can;t do also can;t figure it out & has never seen any thing like it, I am going to try what Rod_Knox suggested to see if it might be the proportioning valve,

Please ley us know what you find out. I am very curious.

Will do ===stupid ten caracter’s.


O K did as you suggested here are the result’s Marked level when cold. I used a sharpi to get a fine line. went to the store 16 mile round trip to warm up not sure if I needed to made a total of 8 stop’s, checked when I got home same level. Mashed the brake hard put level right below line. Waited over an hour back to starting level. level’ Doe’s that tell you if the proportioning valve is possibly going bad?

Everything seems OK. Check occasionally for a few days and if you continue to see the same results I would assume there are no problems.

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Will aboutustly be keeping a eye on it along with every thng else I thank you for your time and shareing your knodgledge with me & everyone else.

Sweet mysteries of life I tell ya… I do like @George_San_Jose1 theory about the rear shoes…as it does hold water. Nice one George :slight_smile:

I can see how an unburped air pocket would lower your level sure…possible… But to make brake fluid? That’s a new one on me… and besides out of adjust rear shoes (over extended wheel cylinder) and or a magical slow to react ballooning rubber brake line (like 3 Stooges style or something) I honestly got nothing for ya… on the making fluid side of things.

Love to know the root of this one…but like the Tootsie Pop question, we will probably never know… Like the Tootsie Pop

P.S… On the other hand…because Georges wheel cylinder theory would be a slow one to manifest… The over extension of the cylinder would eat up fluid (thus dropping level) and its slow to push back in (retract under shoe spring pressure) would appear to make fluid. Same theory also explains the mushy pedal… By-George- I think that might just do it, start to finish.


I hear you this will possibly will be the unsolved mystery I will never figure out.

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Read my edited post just prior to yours… Look above

I saw that.

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There’s a lot to consider. Some seemingly minor issues can add up to a seemingly great issue. But considering the possibility of the rear shoes being out of adjustment and the proportioning valve that is often mounted on the master cylinder port to the rear axle added to the usually clouded reservoir it seemed there might be enough Kentucky wind in the apparent levels of the master cylinder to explain the problem that seems unlikely to actually be a problem. Hopefully.