69 fair lady brake pedal goes to floor

[Dster]

I had decent brakes then suddenly pedal went to the floor. Reserboirs both full, no obvious leaking. With very little knowledge or experience, I figured it was the brake master cylinder. I bought a 260z M/C, bench bled and installed it. I bled all four wheels, starting with R front (mistake) then R rear, L rear and finally L front. We got a little firmness out of the pedal but no real stopping power. Not sure what to do besides another round of bleeding starting with R rear. Any ideas will be much appreciated.
Btw- I had already redone the front calipers and adjusted the rear shoes.
I wonder if I didn’t bench bleed enough or didn’t wheel bleed correctly?

[datsun1500]

260Z is a 7/8 Master Cylinder. It’s the wrong size. You will get nothing on the first pump, plenty on the 2nd with that setup. You need a 3/4 bore Master.

[Dster]

That makes sense, but the MC that was on there was an aftermarket 7/8 bore. The brakes weren’t necessarily super, but they worked fine .

[FergO2k]

260Z is going to be metric flare line fittings, as I am pretty sure Nissan went all-metric starting with 510 (so that would be the assembly line cars starting in '68, ours were really all hand-built).
So metric lines will go in, but not seal well, which is REAL BAD for brakes.
Unless someone unscrewed the large base fittings on the 260Z m/C and swapped in the roadster ones. (I did this when I found a 510 master, way back when, they are hard to find nowadays)

[Dster]

I fiddled a lot with the fittings as I was preparing to bench bleed. I had bought the adapters expecting to need them. To my surprise, the metric ones wouldn’t fit but the 3/8 24 seemed to fit perfectly! I
Now that I’m having trouble I might just take it off and either triple check the fittings or just plop down the $200 for a 3/4 bore at Sincity. I’m reminded of that old definition of a boat - “a hole in the water that you keep throwing money into!”

[fj20spl311]

260Z is a 7/8 Master Cylinder. It’s the wrong size. You will get nothing on the first pump, plenty on the 2nd with that setup. You need a 3/4 bore Master.

NOT CORRECT!!!!!!

A 7/8 master cylinder produces a larger volume of fluid than a 3/4 bore master cylinder.....

[redroadster]

260Z is a 7/8 Master Cylinder. It’s the wrong size. You will get nothing on the first pump, plenty on the 2nd with that setup. You need a 3/4 bore Master.

NOT CORRECT!!!!!!

A 7/8 master cylinder produces a larger volume of fluid than a 3/4 bore master cylinder.....

A larger master causes more effort for the same pressure to the slave though
Do you have a helper to bleed it ?
Unless you bench bled & got all the air out ,you can't bleed 1 side frt then rear
,you need yo open both on the same pressurized the pistons need to travel the full extent

[datsun1500]

260Z is a 7/8 Master Cylinder. It’s the wrong size. You will get nothing on the first pump, plenty on the 2nd with that setup. You need a 3/4 bore Master.

NOT CORRECT!!!!!!

A 7/8 master cylinder produces a larger volume of fluid than a 3/4 bore master cylinder.....

The 1500 uses a 7/8 Master and 1 inch wheel cylinders. The 69 uses 3/4 Master and 3/4 wheel cylinders. With the Master being a larger bore than the wheel cylinders it throws off the bias and causes the exact issue described here. I’ve been through it so I know it’s CORRECT !!!!!!

[fj20spl311]

Your experience my vary, its completely correct about the pedal pressure. There are many things that can throw off the front to rear bias, but changing the master cylinder size is not one of them.

[fj20spl311]

Your experience my vary, its correct about the pedal pressure being greater with a larger master cylinder. There are many things that can throw off the front to rear bias, but changing the master cylinder size is not one of them.

[fj20spl311]

Your experience my vary, its correct about the pedal pressure being greater with a larger master cylinder. There are many things that can throw off the front to rear bias, but changing the master cylinder size is not one of them.

[Dster]

I went ahead and got a 3/4” and was able to install it with no trouble. The line inserts were indeed standard not metric!

[Gregs672000]

Thanks for finishing out the thread. So brakes are good now?

[datsun1500]

Your experience my vary, its correct about the pedal pressure being greater with a larger master cylinder. There are many things that can throw off the front to rear bias, but changing the master cylinder size is not one of them.

I didn’t say anything about the front/rear bias, and as posted going to a 3/4 master fixed the issue. Strange how that worked, huh?

[redroadster]

I the dual master cyl has it built in ,all Datsun cars do, you'll see it bench bleeding ,as the forward ( rears) pushes out fluid 1st ,then the back (fronts ) puts out fluid . The back cyl is tapered to allow this .
Usually the master cyl doesn't really go out all at once , there's a test
To see if it's fully holding pressure
On a scale of 1 to 10 on applying brake pressure with the cyl warm ,push the pedal about a 8-9 , feel if it is dropping for 10-15 sec. Then with out lifting back off the pressure on it to
2 to 3 holding to see if drops ,some minor drop is OK-normal then apply more then back to a 3 ,it will keep dropping if it's leaking , I did the test on most of the cars I worked on, 1000s, as a few I was going to drive the pedal went to the floor on a lite pedal pressure ,the customer says oh yeah you have to pump it twice or more to have good brakes
And once the used MC goes all the way down the cyl, it's seal grabs all the metal fragments rubber trash and is doomed to fail usually when it gets a bit cold