Brand new master cylinder leaking from the lid

[76JimmyBlue]

Hi everybody, having a really frustrating problem here. I replaced the original master cylinder on my '76 K25 with a new AC/Delco one and it leaks from the lid at the gasket. I swapped the bales from the old one and it seems to hold the lid tighter, but still leaks. Either the gasket isn't thick enough or the lid is bent or something. Worst case is the machining of the casting was done poorly. Doesn't look as good as the original M/C, but a straight edge doesn't show any gaps that would cause a leak. Getting REALLY angry at this truck so please, any help is appreciated!

[Getter-Done]

Once the Bail is snapped in, Can you move the lid with your hands?

If so, the bail needs more tension.

If the top surface is not smooth or has machining grooves in it that would cause a leak also.

[Keith Seymore]

I have only recently been having trouble with this, too.

One observation is the rubber gasket is getting all wonky from the fluid. You would think they would use a material that is impervious to brake fluid.

The second observation (from last night) is that the end of the wire bail closest to the brake booster had popped out of location. I put it back in place but it doesn't seem like it wants to stay.

I haven't tried it out yet because I still need to do some bleeding first.

K

[76JimmyBlue]

There is a little movement from the lid with the bails snapped if I really try, but that's ever after swapping them with the ones from the old mc. The lid slid with no real force with the new bails it came with. If I can figure out how to upload a pic I'll post one of the machining of the mouth, it looks suspect.

[LS short box]

Could you "flatten" the top of the bail a bit so you get more clamping force on the cover?

[chevyguyase]

Are you bleeding the brakes by pumping the pedal? If you have trapped air in your system, when you release your brake the fluid will return to the master cylinder rapidly. This will cause the fluid to squirt up against the lid. The air in the system will compress, fluid will not. The fluid should not be high enough to leak out. once you bleed the brakes I think your problem will be remedied. If you ran your M/C empty when you bled it, you may have to re- bench bleed your M/C. You mentioned you still have to bleed the brakes, If you are using gravity, pressure or vacuum bleeding this does not apply to your situation.

[kev2809]

my brand new one did the same thing, used 2 of the rubber gaskets for the lid. mine worked like a charm like this

[76JimmyBlue]

I do get a rush of fluid squirting back out of the master cylinder reservoir. I have a sneaking suspicion that I need to bleed the brakes a little more. I may try swapping the cap from the old master to the new on, already have the old bails on the new one. If that doesn't change anything it may be the casting on the mouth was machined poorly. Will post and update.

[76JimmyBlue]

Here's and update: pretty sure I got the leak fixed. Swapped the old lid and gasket onto the new master, and along with the old bails is creating a better seal. I compared the old lid and gasket to the new lid and gasket, the old one seems to fit the old lid better. The proper solution will be to get a new gasket for the new lid and bend the new bails. For now this works. Thanks for your help everyone!