Master cylinder cap leaking

[Gallop]

I have a 66 with the later style two reservoir master cylinder where the cap is held on with a steel clip. Looks like this one - https://www.npdlink.com/product/master-cylinder-assy-brake/103669/203214

After driving the car, brake fluid leaks out from under the cap. I replaced the cap rubber seal but fluid still leaks out.

Any thoughts on how to stop the leak?

[Clubman7]

Just all the obvious things.
Make sure the surfaces are clean, the cap is not distorted, you aren’t overfilling the reservoir.
I put some silicon rubber grease on my one to stop it corroding around the contact surface.
My rubber seal started leaking after corrosion started making it weep brake fluid.
New seal and cleanup as above and it’s good again.

[Gallop]

Before taking the car out yesterday, I performed a very quick fix by jamming some folded cardboard between the clip and the cap. I did this because on close inspection the clip did not contact the front part of the cap very well. To my surprise, no more fluid leak. I just have to now come up with a more permanent solution. The cap is probably slightly distorted, the new rubber seal may be thinner, the top of the resevoir may have degraded.

The MC has been on the car for 16 years without any issues before. The problem only started recently after I flushed the entire brake fluid. Perhaps the cap was damaged, the clip warped - who knows.