Master cylinder lid leaks, why?
 

I replaced the master cylinder with a rebuilt unit. First time doing this and didn’t realize that I was supposed to bench bleed the master cylinder before putting it on. I have bleed the brakes a couple times with a vacuum type unit. Brakes work great but I’m leaking brake fluid from the top of the master cylinder. At first I thought I had too much fluid, I removed some of the fluid, repainted a few items and assumed I was good to go. I drove it a couple times and seemed to be okay, next thing I know I have fluid leaking again. I replaced the lid and rubber seal with another new one and I’m still struggling with fluid coming out between the seal and master cylinder. Anybody have any ideas what would be causing this? It's getting a little frustrating, ruined the paint and powder coat on a few parts...twice.

Re: Master cylinder lid leaks, why?
 

usually in my experience master cylinder lids only leak if they are over full. Technically you shouldn't need a lid on the master cylinder to use it. It's only to keep moisture out and prevent sloshing. You are only supposted to fill the master cylinder to "full" if you have just installed brand new brakes otherwise the level should be lower because more fluid is in your lines, calipers, wheel cylinders.

Re: Master cylinder lid leaks, why?
 

it could just have a loose unsealed lid, ive seen them squirt two streams up out of the pool of fluid while pumping the brakes, it could be coming out because of a VERY loose lid

Re: Master cylinder lid leaks, why?
 

Here's a pic of the fluid level, it's a little more than 1/2" from the fluid to the top, is that too high? When you flip the bail it seems tight. One thing I noticed while taking the picture, the top of the master cylinder is rough.

Re: Master cylinder lid leaks, why?
 

Does the lid have the rubber (i call them panties) that is supposed to take up room when the fluid level drops?

This needs to be folded up when the MC is full.

Re: Master cylinder lid leaks, why?
 

i would start by putting it together and watching it while somebody pumps the pedal like a madman

Re: Master cylinder lid leaks, why?
 

It does but when you take the lid off they are still at the top. Shouldn't they drop down?
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Re: Master cylinder lid leaks, why?
 

[QUOTE=77BBGMC;4822741]It does but when you take the lid off they are still at the top. Shouldn't they drop down?

No it is working right, the vacume of the fluid level dropping will pull the panties down preventing air getting into the system.

Re: Master cylinder lid leaks, why?
 

You're not alone. My master cylinder(same as yours) would do the same thing. Fluid level didn't matter, it always leaked. I replaced my MC and booster with one from a Suburban that had two individual plastic tanks with lids that snap on tight. Hasn't had a problem since.


Dustyn

Re: Master cylinder lid leaks, why?
 

What year of suburban did you get it from? Did you have to change any lines?

I was told the master cylinder might have air in it. I went through the process last night of breaking loose the lines that feed the MC while my neighbor pressed on the pedal. Went through this process about 6 times per line and nothing changed. It's still pumping fluid to the top of the lid, the MC is only half full and the rubber plungers are not dropping. I think it's time to buy another master cylinder. I hate buying new stuff twice!

Re: Master cylinder lid leaks, why?
 

i bought a new one for my 66 k20 once and i dropped it and they wouldnt installit at the shop cuz it was dropped when i built my 77 shortie i just bought a 86 shortie and used all the good stuff and it had that plastic rsivoir so i thing 85 856 and up have them

Re: Master cylinder lid leaks, why?
 

I got it from a 91. I had to change both the MC and booster because the burb MC wouldn't mount on my booster. Only thing I had to do with the lines was swap the front to the back, and the back to the front on the MC.


Dustyn

Re: Master cylinder lid leaks, why?
 

You said you replaced the lid with another new one. Was the first one a new one? Maybe the lid is warped. I bought a chrome lid once, and it would not even go on the mc.

Re: Master cylinder lid leaks, why?
 

I just went through this myself. I cleaned the gasket real well, put a small bead of rtv around the gasket on the side that will contact the master res and presto, no more leaks.
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Re: Master cylinder lid leaks, why?
 

I installed a new master cylinder last night, did the bench bleed on the MC and once installed bled the breaks. It was late when I finished and I could tell I still had some air in the lines. I drove it around the neighborhood and when I got home double checked to make sure no leaks on the MC, all seemed good. Tonight I re-bled the brakes and was struggling to get the air out, I used a hand held vacuum unit and did it the traditional way. I started it up to move it and my brake pedal went to the floor and the brake light stayed on. After that I couldn’t build any pressure in the lines. I’ve pumped the brakes countless times with no luck. Would it be possible that something happened to my brake booster while I was bleeding the brakes? I noticed something odd, the front of the booster expands (not a lot) when the brake pedal is pushed in. Any thoughts?

Re: Master cylinder lid leaks, why?
 

It sounds like you got a bad master cylinder, I personally would take it back and demand another.
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Re: Master cylinder lid leaks, why?
 

Thanks guys for all the feedback. I guess the third time is a charm, I don't know if it was something I was doing or the rebuilt master cylinders. I'm just glad to have brakes with no leaks.