Just for grins, Why not remove the proportioning valve and plumb it back to stock. Take that variable out of the equation. See what happens and go from there.

Edited May 15May 15 by Yarb

A couple more things to consider, tools for problem solving:

Air in the hydraulics can be "pumped up" to get the brakes to work. Each pump compresses the bubble(s). If you're just getting a pedal that drops and pumping doesn't help, that's a seal problem, not an air problem. Unless it's a really big bubble. I had a big bubble from switched calipers. It's burned in to my brain. I could lock up the backs with pumping but the fronts were useless.

And, a leak at the back of the master cylinder will not be visible until it drips down the firewall or comes out of the bottom of the booster. Don't drip brake fluid on your paint. It leaves a mark.

Edited May 15May 15 by Zed Head
- a

Okay, here’s my status after fixing Thursday night’s fiasco of swapping the lines and then swapping them back to their correct positions:

I verified that all of the lines are correctly routed. Forward reservoir goes to aft port on the pressure sensor block, then to the proportioning valve, then to the rear brakes. After reservoir goes to forward port on the block, then to a junction, then to the front brakes.

I disconnected the MC from the booster and checked for leaks. There is no brake fluid or other residue in there.

I also checked the push rod length and made sure it has about 1mm of clearance between it and the back of the MC piston, and that it is tightened down.

I checked the brake pedal height. The face of the pad where your foot touches is 7.5” from the carpet on the firewall. This exceeds the 5.8 to 6.2” I’ve seen recommended, but I’m not sure where that measurement is supposed to be read from. Is it the back of the pedal, the face of the pad, or somewhere else?

The brake pedal has almost exactly 3” of travel, 1” of which is free play. Red flag. The pedal does not contact the pad on the firewall when fully depressed.

I bled the MC at the side bleeders with the caps off, running that fluid back into the reservoirs with hoses.

Then I put 44 fl oz of brake fluid into the pressure bleeder, connected it to the reservoir for the rear brakes, and pumped it up to 15psi per the instructions. I let that sit for 15 minutes and went around the car looking for leaks. I didn’t find any, so I checked the pressure. No change.

I bled the left rear brakes until I had zero air, allowing about a pint of brake fluid to pass through the system.

I filtered the fluid, put it back into the bleeder, brought it back to 15psi, and did the same to the right rear caliper.

I repeated this sequence on the right front and then the left front.

After all of this (and with the engine off) the pedal is firm, but the amount of free play I have is concerning.

I tried getting rid of the slop by increasing the length of the pushrod, but it persists even when I know the push rod is too long and is pressing the piston as I tighten the MC down, preloading the MC. I put it back to 1mm of clearance for now.

I’m thinking this might have been my issue all along. I’m not sure if the pedal travel spec included the free play or not, but if not then I don’t have enough pedal travel. Either way I need to reduce the free play by at least 0.5”.

I’m not sure why there’s so much free play in the pedal. The pivot bushings are brand new and snug. I’m going to get under there and see if it’s at the clevis today. Maybe the pin is too small and needs a bushing. I know the pushrod moves freely in and out about two millimeters if you push and pull it with your fingers. I assumed that was slop at the clevis. I don’t thing that would translate to half an inch at the pedal, though.

Edited May 16May 16 by Matthew Abate

You have definitely gone through the system piece by piece. Hopefully you’ll get some feedback on the pedal travel.

46 minutes ago, Yarb said:

You have definitely gone through the system piece by piece. Hopefully you’ll get some feedback on the pedal travel.

Hopefully. It feels like I have competing problems.

Not enough pedal travel would suggest the MC is preloaded and the pushrod needs to be shortened.

Too much free play suggests the pushrod is too short and has too much gap to make up before it starts working.

I was originally leaning toward the latter because this could also cause travel too be too short if something else is stopping the pedal before the MC is fully engaged, explaining the weak brakes, but I can’t figure out what that would be. The pedal isn’t contacting the pad and I don’t think the spring is fully stretched. I don’t think there’s anything else in there that could cause interference.

Edited May 16May 16 by Matthew Abate

When you started mentioning the brake pushrod, my mind went to the reaction disc. If it has dislodged you would have some free play there.

Just another thought.....

I thought the same thing this morning. Another thought, I f the disc has fallen to the bottom of the booster would that prevent the rod from making a complete full cycle in the master cylinder. Just a thought at this point you’ve certainly checked everything else.

I would think pressing on the master cylinder is not a problem until the ports to the reservoirs are covered, preventing retur flow. Someone can correct me if necessary

1 hour ago, Yarb said:

I thought the same thing this morning. Another thought, I f the disc has fallen to the bottom of the booster would that prevent the rod from making a complete full cycle in the master cylinder. Just a thought at this point you’ve certainly checked everything else.

God, I hope not. How would I find out? I’m guessing pull the booster out and possibly open it up.

I bought it new from Z Car Depot but about seven years ago. It would have to have been a faulty booster out of the box.

My dad thinks the booster pushrod is not long enough, because I have the clevis all the way at the end. I’m skeptical but it’s possible, because it’s not a Nissan booster but some aftermarket thing.

I’m going to mess with the pushrods and pedal position tonight to see if I can adjust some of this stuff out of the equation.

To check it you would need to pull the master off. Then dig the spring/locking washer out. Carefully pull the push rod / piston out and see if the round disc is still on the rod or in the hole where the plunger sits.

If it has fallen off you then have to pull the whole booster and shake it for confirmation of the disc being in there. If it is keep shaking and check the plunger hole till you can see it and grab it with needle nose pliers. I glued mine to the plunger to avoid future issues.

When in the proper place it won't move. just when the plunger moves outward do these issues occur. I had a rebuilt booster from MSA do this before installation and it took a while to figure out. The pushrod "nut" on my new booster was too short and I had to swap out my old long "nut" to even get the rod to reach the M/C. You can see the longer "nut" in this picture, as well as how the reaction disc sits (or falls off).

I hope you can get this resolved.

Here's a tool for setting booster rod length. I just picked one site, they're all over the internet. Search "booster master cylinder tool".

D and C Extreme Industries

Master Cylinder to Booster Measuring Tool | D and C Extre...

SKU# Wil 260-18027 Shipping $8.00

6 hours ago, Matthew Abate said:

God, I hope not. How would I find out? I’m guessing pull the booster out and possibly open it up.

I bought it new from Z Car Depot but about seven years ago. It would have to have been a faulty booster out of the box.

My dad thinks the booster pushrod is not long enough, because I have the clevis all the way at the end. I’m skeptical but it’s possible, because it’s not a Nissan booster but some aftermarket thing.

I’m going to mess with the pushrods and pedal position tonight to see if I can adjust some of this stuff out of the equation.

When you received the booster was the rod inside the booster secured to the master cylinder mounting studs with rubber bands? That procedure keeps the rod and the disc firmly in place preventing it from being dislodged and falling to the bottom.