Okay, as usual it ended up being something stupid, but I’m not 100% certain it wasn’t also air in the system.

This morning I got in the car and the brake pedal was firm, so no air, right? Just wait…

In order to confirm my pedal was not bottoming out prematurely (I only have 2 inches of pedal travel) I pulled the master cylinder off the booster and measured the pedal travel again. Or rather, I didn’t have to because it went to the floor.

Okay, so that confirmed that the pedal travel is not being determined by the booster or something under the dash fouling it.

Next, I reconnected the MC and bled it via the bleeders on the side. For one, I got some air out of the rear bleeder (for the front brakes). Also, the pedal went to the floor. So this confirmed there is nothing physically obstructing the piston movement inside the MC (assembled incorrectly, broken spring, whatever).

Next, I checked the pedal travel with everything closed up while bleeding the right front caliper. It went to the floor. I also got a fair amount of air.

So now I know for sure that the pedal travel is being determined by the available space for fluid (i.e. the caliper piston stroke).

Next, I bled the whole thing with the power bleeder (left rear, right rear, right front, left front). I let a full quart of fluid pass through each caliper before moving to the next, which is more than double what the lines and calipers hold. I got zero air from any of them.

I checked the pedal travel and it was back to 2 inches.

Then my buddy who was helping asked me if the reservoirs were over filled.

Ding ding ding!

They were above the max fill lines by a quarter inch, so I removed fluid until they were at the line. Now I have 2.25 inches of travel.

I took it for a test drive and was able to lock up the rears at ten, twenty, and twenty five mph, both on flat ground and down hill.

The brakes still don’t inspire a ton of confidence, but that is likely due to the proportioning valve biased too much toward the rear brakes. I plan to adjust this properly on Sunday in a large empty parking lot. I’m also going to pull out some more fluid so the reservoirs are not right at the max fill lines to see if that does anything of note.

That was a painful way to learn what happens when you overfill the fluid.

Edited June 11Jun 11 by Matthew Abate

I’m inclined to think that it was air in the system. Can’t see where over filling would be an issue as long as the Mc has sufficient fluid to work properly. My two cents. Either way I hope you have this wrapped up.

The reservoirs being over filled is a red herring. No effect on how the pedal feels.

Let's hope it was air in the system that you got out!!

I had ask d Silvermine Motors for advice a while back because they sell some of the bits I’m using, and they suggested today that it could be because I installed the residual pressure valves that came in the rebuild kit. Apparently they are only meant for drums and should be removed / left out on the ports feeding disc brake calipers.

I also just saw a video from Summit Racing saying that a larger diameter MC means less pressure and a harder pedal, reducing clamping force. I would have thought the lines would determine that, but I guess a given amount of force spread over a larger area could cause that.

Edited June 12Jun 12 by Matthew Abate

Didn’t the 79/80 ZX come with rear disc? I had the same conversation with Edan quite awhile back. He referred to them as check valves.

The 79-80 280zx came absolutly with rear discs (and ventilated front discs) so, discs all around.

I mentioned that the ZX had four wheel disc brakes and his response was that the rebuild kits and new replacement MCs have them but they weren’t used from the factory. I don’t know if that’s verifiable. There’s a discussion on hybrids about this and I would say it’s inconclusive.

I dug around and found a number of forum comments saying they should be removed when using discs in other cars (corvettes, hot rods, etc.).

Wilwood says to use them when the MC is mounted below the caliper. They recommend 2 to 4psi for disc and 10psi for drums. However, there are inline valves, not components in the MC. They don’t say anything about not using them when the MC is mounted higher than the caliper.

I don’t think this should effect the clamping force of my brakes, though, because they only restrict flow BACK to the MC.

I could just see what happens if I remove them. It’s a zero-cost change that I can reverse if it doesn’t help. If that doesn’t work I’m going to have to revert to stock in the front.

Edited June 12Jun 12 by Matthew Abate

Would be nice to see a diagram of a factory ZX master cylinder to verify if the check valves are there. I have 3 aftermarket units and all 3 have them.

Edited June 12Jun 12 by Yarb

1 hour ago, Yarb said:

Would be nice to see a diagram of a factory ZX master cylinder to verify if the check valves are there. I have 3 aftermarket units and all 3 have them.

46010-p6500:

46010-E8702:

280zx RPV parts (Nabco):

There they are #14

Do the front discs show signs of wear at all? Your lock-up experiments suggest that there's no front brake action. At ten mph it should have stopped immediately just from weight transfer to the fronts. A bad seal on the piston that provides front brake pressure could do that. The MC is two pressure cylinders in one bore. Might be that your front brake piston is just along for the ride in the MC bore.

22 hours ago, Matthew Abate said:

able to lock up the rears at ten, twenty, and twenty five mph

Yes, they are grabbing, just poorly. I did have the proportioning valve fully open, which I think means the rears are set to 50/50. I can’t spin the wheel by hand when it’s up on jacks and someone steps in the pedal, but that doesn’t mean much beyond there is some pressure going to the front.

Edited June 14Jun 14 by Matthew Abate