The cap on my 380SL is the same as the caps on the master cylinders of my Volvos (240, 740 series). I have extras from parts cars. I drill a hole in the center of the cap and then slowly enlarge it until the schrader valve pulls tightly into place . The vent system built into the Volvo caps requires a piece of the underside of the cap to be removed and the rubber gasket is then very loose but if you are careful it isn't a problem (I supposed I could cement it into place). I never use the brake pedal to bleed brakes anymore in any of my cars. As soon as the seals on the master cylinder travel farther than normal down the bore there is a great chance that rust or build up in the lower reaches of the bore will damage the seals and then you are looking at a new master cylinder.