Brake fluid from master cylinder lid

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Frankenchevy

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K30 with hydroboost, new master cylinder. Have brake fluid seeping past lid. Is it just over full? Maybe the lid is just janky. You can see in second pic it’s making it onto the mating surface. Retainer clip feels solid. Second pic shows fluid level even after leakage... it’s still plenty high.
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Preston

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Could be the gasket may have a bad spot in it, turn it around and see if the leak moves, just a thought have a good day.
 

78C10BigTen

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Mine kinda has a wetness like that too. Never really gave it to much thought. Leaks everything else so wats a lil brake fluid. You got some damn clean fluid though. Also pretty sure you can buy a new rubber in the help! section at the auto store.
 

Frankenchevy

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Could be the gasket may have a bad spot in it, turn it around and see if the leak moves, just a thought have a good day.
Good thought, but the lid and gasket are directional.

Mine kinda has a wetness like that too. Never really gave it to much thought. Leaks everything else so wats a lil brake fluid. You got some damn clean fluid though. Also pretty sure you can buy a new rubber in the help! section at the auto store.
Entire brake system except hydroboost pump and rear calipers are new, hence the clean fluid. Maybe I’ll take a spin to Napa where I bought the new mc and show them the leak and they can give me a new gasket for free. 30 year old mc never gave me trouble. I just replaced it because I was doing everything else...
 

idahovette

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If you still have your old cylinder, clean the cap and the rubber gasket and try it on the new one
 

HotRodPC

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That engine compartment is to damn clean anyway. Let that **** leak.

I'm thinking bad gasket, maybe warped or bend in the lid? I highly doubt it's the cast iron, but you could always run a straight edge on top of it to be sure.
 

sdewolfe

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The cap/bail/bellows/gasket depends on low pressure on both sides of the gasket to maintain a seal. If pressure builds inside the reservoir, the gasket will not hold back fluid. Excessive back pressure is caused by air in the system or a defective seal at the working end of the MC piston. The first opportunity for pressure in the wrong place is when you depress the pedal. Protect all the painted surfaces you want to keep that way. Remove the cap and watch the surface of the fluid in the reservoir while someone presses the brake pedal for you. If there is a geyser during brake application stroke, the seal on the piston is bad. Continue watching the fluid surface as the brake pedal is released. Does it geyser now? If so, there is an air bubble trapped in the system. You could have one or both problems. If the rubber seal is bad, the person pressing the brake will report the pedal is sinking and you may see a continuous welling-up of fluid while the pedal is moving. If brake pressure holds at full stroke, but there was a geyser during the stroke, there is a scar in the bore; once the rubber seal travels past the scar, it seals the bore. Just because the MC was brandy-new-rebuilt doesn't mean it is good. :)
 

Frankenchevy

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The cap/bail/bellows/gasket depends on low pressure on both sides of the gasket to maintain a seal. If pressure builds inside the reservoir, the gasket will not hold back fluid. Excessive back pressure is caused by air in the system or a defective seal at the working end of the MC piston. The first opportunity for pressure in the wrong place is when you depress the pedal. Protect all the painted surfaces you want to keep that way. Remove the cap and watch the surface of the fluid in the reservoir while someone presses the brake pedal for you. If there is a geyser during brake application stroke, the seal on the piston is bad. Continue watching the fluid surface as the brake pedal is released. Does it geyser now? If so, there is an air bubble trapped in the system. You could have one or both problems. If the rubber seal is bad, the person pressing the brake will report the pedal is sinking and you may see a continuous welling-up of fluid while the pedal is moving. If brake pressure holds at full stroke, but there was a geyser during the stroke, there is a scar in the bore; once the rubber seal travels past the scar, it seals the bore. Just because the MC was brandy-new-rebuilt doesn't mean it is good. :)
I’ll try these, thanks! Should I have the engine(hydroboost) running?

The master doesn’t seem to seep when not running.

The MC is new, not rebuilt. But I understand manufacturing isn’t 100% perfect.

Edit: and by geyser, do you mean a slight welling up or should I see fluid squirt from the surface of the pool?
 
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Dutch Rutter

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I just had this same issue and fixed it up this last weekend. bought a new MC/Vac booster 3 years ago. Damn things leaked from the cap as well as between since day one. After finding my rear rez empty I decided it was time to replace them again. Got all new stuff via lifetime warranty and installed it to find the brand new MC cap to be loose, bent the lid keeper bar some to hold tighter and so far so good.

Good luck!

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Frankenchevy

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@sdewolfe , there must be a scar in the bore. The pedal is firm and never lets off. I watched as a helper slowly pushed the pedal and fluid shot up from the front(smaller) reservoir, but the pedal held firm at the stopped position. We held it there for quite some time with no further travel/sinking. Upon release, no movement in the surface of the fluid was observed.

This was all observed with the engine off. Great luck with all these new parts. I guess I’ll pull the MC and head back to Napa to swap it out. What a pita.
 

Frankenchevy

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Round 2
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Frankenchevy

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Everything looks perfect after a 20 mile cruise.
 

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