okay, progress but not full resolution.

I readjusted the pushrod on the engine side of the booster using a home made version of the tool Zed Head posted above. Then I went under the dash again and measured free play (now 0.5”) and travel (almost exactly 2.0”).

The brakes are still weak, but the pedal is firm. I’ll take it for a longer test drive in the morning to see if anything else is going on.

I have no idea if the pushrod was secured when I got it. The purchase was years ago, and I think I installed it into the car pre-Covid. That’s how slow this build is going.

Edited May 17May 17 by Matthew Abate

Hey my friend I’m on year 6 and finally closing in.

43 minutes ago, Yarb said:

Hey my friend I’m on year 6 and finally closing in.

Got you beat by a decade...

Edit: I have a new check valve coming in the mail, just in case.

So the pedal is still firm after sitting for a few days. Right now I think there is a relationship between the weak brakes and the pedal only having 2 inches of travel. I think it’s supposed to be 3.5 inches. I can’t think of what would cause both of those issues at the same time, though.

I used a NOS rebuild kit on the master cylinder, so I don’t think it’s the relief valves, as someone mentioned earlier. I’m pretty sure I checked the block with the pressure sensor that’s on the inner fender and it was clear of debris. There’s nowhere else for the front brakes to have restriction because all of the lines are brand new and the calipers were fresh out of the box.

I also don’t think it’s a capacity issue. I’m using the 15/16 version of the 280zx MC, which is supposed to be enough.

So what would cause the pedal travel to be too short? I don’t see any interference under the dash. I can’t tell if the pushrod is bottoming out inside the booster or not. The pedal just stops.

Edited May 21May 21 by Matthew Abate

2 hours ago, Matthew Abate said:

rebuild kit on the master cylinder

There are many parts in there. Considering the time you're spending on it, it might be worthwhile to try a parts store MC just to be sure it's not that. Although, they are spendy.

https://www.oreillyauto.com/shop/b/brakes/brake-hydraulics/brake-master-cylinder/3bff094046ed/v/a/16441/automotive-car-1973-nissan-240z?m

Okay, I’m throwing my hands up. At this point it’s voodoo.

The new check valve made no difference, plus the pedal is soft again after sitting since my last post.

The only thing I can think of is there is air in the system somewhere. I expected all this bleeding, especially with the power bleeder, to have taken care of that, but I have no idea at this point.

No money to take it to a shop or buy anything, so I have to figure it out with what I have, which means I’m going to take a break and let it sit for now.

Are you just assuming that your rebuilt MC can't be the problem? All of the other parts are factree except that one, if I read things right.

Depends on what you mean by factory. Nothing is original except the pedal box.

I have an off brand booster from Z Car Depot with an OEM check valve (the hoses are clamped) and push rod, a 15/16ths Nabco 280zx MC, a Wilwood manual proportioning valve, stainless hard and soft lines from Z Car Depot, ‘84 Maxima rear calipers, and Toyota S12W front calipers.

The MC rebuild kit was NOS and made by Nabco. The soft parts were in good shape, so I used them. Right now I think if it is the MC it’s either because the seals are leaking or that I over honed it and it’s not forcing the fluid down, but it’s not leaking out the back, so I don’t think it’s a problem with the hone.

It’s possible it still has air in it, but I’ve bled it at the bleed screws a few times.

One thought I had this morning was maybe I have air trapped in the caliper cylinders, but I have them on the correct sides and the bleeders are pointed up, so I expected the power bleeder to take care of that. The pads on all four corners are skimming the rotors without pressure more than I would have expected, so maybe something is up with that. I assumed that was just because they are fresh out of the box, though.

I have not read anything about this MC being too small for this combination of calipers, but maybe that’s part of it. As far as the brakes being weak, I can’t evaluate the cause of that until the pedal stays firm.

I have the same setup except I have 240sx calipers on the rear No wilwood valve

9 minutes ago, Matthew Abate said:

Right now I think if it is the MC it’s either because the seals are leaking or that I over honed it and it’s not forcing the fluid down, but it’s not leaking out the back, so I don’t think it’s a problem with the hone.

Your problem describes air leaking in, not fluid leaking out.

One thing you might try, if you're actually seeing air released whenever you bleed it, is to bleed at the MC using the bleed ports there, intead of at the wheels. If you get air from the MC then you can refocus there.

One concern is the stainless lines. They are notorious for being hard to seal. Maybe the air is leaking in at a joint?

I originally bought a set of stainless lines. Couldn’t keep them from leaking. They sent me a set of steel lines. Not great but was able to stop the leaks. Not saying that’s your problem but was a pain for me.