Master cylinder lid seal 68 dart
I have often found the cause to be a violent return of fluid to the reservoir caused by poor brake shoe adjustment with the springs returning
the shoes an excessive distance, but most often air in the brakes that compresses when you apply the brakes, then violently shoots fluid BACK in a fountain/geyser when the pedal is released.
The cap was designed to control sloshing around, not a forced jet of fluid.
To check, have someone pump the brake pedal a coupla times and hold it down hard.
Remove the m/c cap, watch the fluid , and have the pedal released.
There should be slight fluid return to both systems .
I suspect one will have a heavy return that, if spashed on the paint, will ruin the paint, rinse with water IMMEDIATELY ! !
Adjust and/or bleed the system with the heavy return .
Good luck .
Cheers