I'm replacing the master cylinder on my '84 GSL (OEM 4-wheel disc). The rebuilt master cylinder I received (on right in photo) doesn't have a check valve in the rear port (forward most port on install cylinder. The one I'm replacing had been replaced by the previous owner and has a check valve at the outlet port for the rear circuit (cylinder on left).
Which one's correct for GSL? With or without rear checkport?

 

Can you see a check valve or ball inside the port? If not, my guess would be the bottom port was stripped at one time and when it was rebuilt, the rebuilder drilled and tapped it for a fresh fitting. I have not seen that extra fitting on any 1st gen.

 

The "clean" fitting on the left cylinder, has a small rubber diaphram with spring under it. I heard that the 1st gen's with rear drum brakes had a check valve. Is the check this a rear drum master?