the disc drum one i pulled apart had the same check valves front and rear and i ran it with disc disc setup with both check valves in with no problems. brakes never dragged

but why did you buy an 11/16th master to run with 4 piston fronts and disc rears....why not go something a little bigger. from what i remember the larger diameter ones weren't any more expensive

Have a look at the valve
If its the 'duckbill' style you should be able to tell the difference between them by looking at the sides

The drum one will be round... while the disc one will look similar but have 2 thin grooves down the side
These allow the fluid to drain back

Essentially a disc valve lets more fluid back than a drum valve.... some masters have no valves...some incorporate disc valves rather than none at all... makes them alot easier to bleed up when theres still about 2lbs of line pressure

Apparently Datsun fitted some a very low pressure check valve. It is common.

Just replacing my master cylinder

i bought the JB1196 (PBR Model) to suit 120y disc front drum rear with no booster (11/16th size)

I have 4 wheels discs though (hilux 4pot front, bluebird disc rear)

i knew about removing the check valve, so i pulled apart the two bottom sections of the cylinder with the intention or removing the check valve from the rear brake circuit (to suit rear disc)

out of curiosity i also opened the front circuit, and was surprised to see a check valve in there

can any one explain?

ATM im going to push on just removing the check valve from the rear circuit as has been advised but im a little confused by my finding