after reading this thread from the tech section
http://datsun1200.com/modules/myalbum/photo.php?lid=1399

im kinda confused as what to do when fitting discs, as i have just pulled my master cylinder apart and found that it has the "disc" check valve in both fron and rear
the dics one apparently has 2 small grooves down the side of it which both of mine do

the confusing thing is that it is supposed to be a dics drum master and it still holds pressure on the front, well it did till i disconnected the front brakes as they were making it hard to push around the shed

so im wondering if it would be a good idea to remove the check valve front the front circuit as i still have rear drums? and would i remove both if i was to fit discs all round?

ahhhhh ..... saved me typing the same question any ideas guys ??

I can answer this.

Discs use 2 psi residual pressure.
Drums use 10psi residual pressure.

Now this is why there are drum master cylinders and disc master cylinders. Or more commonly a mix of disc/drum master cylinders.

You can buy the expensive wilwood in-line residual pressure valves, and remove the ones in the master cylinder, or you can use a factory one that's appropriate for your disc/drums disc/disc configuration.

So when you do a 120Y brake upgrade from a 4 wheel drum setup, you should use the 120y master cylinder (Preferably from the earlier non-boosted 120Ys).

kinda makes sense, but realistically wouldn't all master cylinders that are designed for a disc/drum application have valves internal designed to hold the correct psi in the line regardless of the caliper on the end ?? or does the actual caliper have some influence in the amount of psi the master cylinder will hold/release ?

yes. All master cylinders have some form of residual pressure valve integrated.

But they are specific to discs or drums. I once recommended removing the residual pressure valve, I'm less likely to recommend that today. Getting another spring rubber valve thing from another master cylinder's front circuit would be the best advice.

My rear brakes hardly worked at all, Not that the car has been driven much. I haven't even bedded in the full width of the brake pads yet.

well the thinkg that has confused me is that the front and rear pressure doovy are both the same in my 1200 coupe master, disc drum
and i also pulled apart a 120y master and they were both the same in that as well

thats the thing that has stumped me. why are they the same if one is for disc and one for drum

PS really hard to type when drunk

I didn't look that close, but the springs might be different tensions, or the hole into the master cylinder might be smaller. While you have it apart (if the beer is not affecting you too much) have a look at that.

I know of loads of people who didn't do anything about these things, and didn't have any problems.

What do all the kids with 1600's do when they upgrade to R31 rear discs?

For my rear discs I drilled a very small hole in the original valve to relieve residual pressure. As per what Dattodude said, perhaps this was the wrong things to do. Maybe I'll also find the rear brakes not doing much?

you know I just had a thought but it probably not worth much investigation. What if there is enough room in the master cylinder assembly to drill out that hole large enough and plug it up with a carby jet. That way you can change the pressure with the jetsize?Quote:

daveman was talking about drilling the rubber valve. Not the master cylinder.

I don't think it's a good idea to play around with the master cylinder too much.