Master Cylinder Reservoir Cap Gasket?

[sinstar]

I need a replacement master cylinder reservoir cap gasket for my 70 Duster 340.
(manual brakes, front disk/rear drum)
The one in place now (leaky) has the part #20872 stamped on it.
Anyone know where I can purchase a replacement?

 

[nm9stheham]

Is this a Bendix part number or a Mopar? Mopar should have 7 digits.

 

[Inertia]

Done any brake work lately.?

 

[sinstar]

Just a fluid top-off. Why?