The lines going from the MC to the block with the pressure sensor are fitted correctly, and I always bleed the car starting from the left rear, to the right rear, right front, and left front.

I’m going to try bleeding it again to see what happens.

Edited May 13May 13 by Matthew Abate

On 5/12/2026 at 9:22 AM, Matthew Abate said:

I can’t get the system completely empty of air. Every time I think I’m good, I come back the next morning and the pedal is soft (I usually do this at night so I don’t drive immediately after I finish). There are no leaks.

Can you clarify a little bit about the order of events?

You drive the car and the brakes are soft.
You get home and bleed the brakes and immediately after bleeding they feel good solid firm.
You go to bed and get up the next morning, and the brakes are spongy again after sitting static overnight.

Is that the correct order of events?

9 minutes ago, Captain Obvious said:

Can you clarify a little bit about the order of events?

You drive the car and the brakes are soft.
You get home and bleed the brakes and immediately after bleeding they feel good solid firm.
You go to bed and get up the next morning, and the brakes are spongy again after sitting static overnight.

Is that the correct order of events?

Pretty much. I bought a pressure bleeder and I’m hoping it will be the silver bullet. I am CERTAIN I don’t have a leak. I’ve gone over the entire underside of the car and checked every junction multiple times while my buddy pumps the brakes. There’s no residue anywhere, and the reservoir levels don’t change over time.

There has to be a bubble somewhere in the middle of the system, maybe before the proportioning valve, that just won’t move. I can’t think of anything else, and I can’t evaluate the effectiveness of the calipers until that gets resolved.

Edited May 14May 14 by Matthew Abate

Well then I'm not sure a trapped air bubble would explain your order of events........

You're thinking it's a trapped bubble that never goes away. Problem is, if it's always in there, then how would the brakes ever feel correct? Even right after bleeding? If the bubble is still in there, the brakes should never feel correct firm even right after bleeding.

Doesn't necessarily line up with the order of events.

So what’s another reason I would have a stiff pedal after bleeding that gets soft after sitting but no fluid anywhere under the car? If it’s not trapped air, fluid has to leave the system to introduce air, right?

Edited May 14May 14 by Matthew Abate

Well, a couple thoughts... First is that I disagree with the premise that fluid has to leave the system in order to introduce air. If you somehow create a vacuum in the system, you can introduce air without losing fluid. Zed Head brought up the idea of system vacuum earlier, but I do not believe that his suggestion about the caps on the master would cause that condition. They are designed specifically to have that NOT happen. So I'm struggling with coming up with a way to create a vacuum in the system which makes this portion of the discussion an academic exercise. In other words... a vacuum could pull in air without losing fluid*, but I have no theory as to how it could apply to the current situation. Haha! 🙂

So trying to come up with a theory that supports the "trapped air" premise, I could possibly be convinced that it IS in fact trapped air, and maybe the residual pressure valves (which may be built into the master cylinder??) keep said air bubble "compressed" for some time after bleeding. Goes like this:

You bleed the brakes. When you press down on the pedal, the first thing that happens is you compress a trapped air bubble. This compression reduces the volume of said air bubble to way smaller than it was when you started. Then the residual pressure valve(s) in the M/C keep this bubble somewhat compressed, even after the bleeding process is completed. But then slowly (because most valves are not a complete/total hermetic seal) the pressure bleeds back past the residual pressure valves and that bubble re-expands to it's "atmospheric" volume.

Only problem with that theory is that the first time you hit the brakes, it would re-compress the bubble. But that's all I got right now.

* I've seen it happen on wheel cylinders and clutch slave cylinders where the seals are specifically designed to hold fluid in, but when stressed in reverse (vacuum), they don't do a good job of keeping air OUT under some situations.

I’m sure you bench bled the MC before bleeding correct. Even if you didn’t, as much bleeding you’ve done so far I would think that air would have escaped at this point. Definitely a strange one. Has the bleed machine arrived yet?

Edited May 14May 14 by Yarb
Spelling

I figured it out. I have the proportioning valve connected to the wrong line.

Edited May 14May 14 by Matthew Abate

On 5/12/2026 at 6:22 AM, Matthew Abate said:

Toyota brakes in the front and the maxima disc conversion in the back

I didn't bring this up in the beginning because the focus seemed to be on the overnight change. But, as far as the general bleeding procedure, it's been described in the past that some of the conversions don't put the bleed port inside the piston bore at the top, where the bubbles are. People have had to unbolt the caliper and tilt it to get the port in the bore at the actual top.

You can probably see the casting ridge where the channel from the bore to the bleed screw is. Make sure that the exit from the bore is at the top when you bleed them.

In short, just because the bleed screw is not at the bottom, it does not mean that the orientation is correct for bleeding.

From Rockauto 1984 Maxima rear. Not sure what you have.

From Rockauto 1989 Toyota pickup.

16 minutes ago, Matthew Abate said:

I figured it out. I have the proportioning valve connected to the wrong line.

That’s fantastic, hopefully you don’t have to make a new line.

2 hours ago, Matthew Abate said:

I figured it out. I have the proportioning valve connected to the wrong line.

Awesome. Hope that's the smoking gun and the problems all go away! I was already struggling to come up with scenarios of what could cause your problem, so I'm hoping we're done with that! Haha!! 🙂

Well, looks like I’m making stupid mistakes because I’m tired. After I got the lines swapped I realized that they were not wrong in the first place and now they are. Back to work.