Well, here's the story:

After a weekend of Auto X duties, I drove home, on the highway, after a 20min+ cruise with very little/no brake applications, I went to hit the brakes to slow for oncoming traffic, and the pedal went almost to the floor. I pumped it and got some brake (thank goodness).
I checked everything I could and found nothing, the pedal returned to it's normal stiffness and drove fine afterwards.
then after another 15-20+ minute cruise on the highway it happens again.

This issue disappears after a few brake applications, and is virtually non-existant until I cruise on the highway for a while without applying the brakes for a decent amount of time.

I'm scratching my head on it, but here are some thoughts:

- ABS modulator somehow bypassing internally, for whatever reason only doing it after a long highway cruise
- Master cylinder bypassing (this is hard to believe, otherwise it would do it all the time, right?)
- Brake fluid is boiled, causing a problem somehow? I have no clue how this would cause a problem, or why
- Some part of the brake system pulling air in (also hard to believe, otherwise it would happen all the time)
- Seized front caliper pins (they are tight, i've checked), if there was enough wheel runout or rotor runout, long drives could push the pads away a good amount, and seized pins could cause the pads to not want to go back in on a brake appliaction, however, this (in my mind) SHOULD cause a HARD brake pedal, which is not what I"m getting.


Does anyone have any other guesses? Right now I'm debating whether I should replace the master cylinder as a guess, and hope for the best. I'm also going to plan on replacing the front calipers for G35 or 6th gen calipers at some point, but I have a hard time believing that is the problem.

 

Brake booster leak possible?

 

Ding ding ding!

Bleed your brakes and use higher performance brake fluid like DOT 4 or 5

Our standard is a rather low DOT3.

 

I thought about a booster leak but that would cause a HARD pedal, right?

Can't put DOT 5 in, not compatible, DOT 5.1 is though. I'll try some DOT 4 I guess, I have a power bleeder I still haven't used so it's worth a shot I guess! But how would boiled fluid cause this to happen in such a strange situation only? It literally never happens in town or on short drives, only after a long highway cruise with no brake appliaction.

 

brake booster leak gives you no power assist, as in you gotta press the pedal down pretty hard to get brake application

 

^ These were my thoughts but I could be wrong.

Give the fluid change a try. It's the easier of the two to do.

 

This only happens in hwy driving? How bout city?

Edit* Also has your fluid level changed by any chance? that you know of?

 

it was a common thing with Datsun truck guys, brake booster put alot of vacuum strain on the old 4 banger engines so some guys deleted the brake booster. ive driven without a booster myself and i dont need giant muscular legs from smashing the pedal down so i hooked it back up

 

That's what I'm saying, the pedal goes SOFT, as if there is a brake fluid leak (but there isn't).

And yes, it ONLY does it on the highway after driving without braking for a fairly long time, the longer the period of driving without braking, the worse it gets. I almost hit someone the other day because the pedal literally went within 1" of the floor before I got ANY pressure at all.

 

im trying to think because to me the booster should be a constant problem, maybe the primary or secondary seals in the master cylinder are bad? if one of those are bad i can see it being an off and on problem i suppose. tough to tell

 

Yeah it's a messed up issue that is for sure! I can see the booster being an issue on the highway because you are driving under higher load (less vacuum) on the highway for the most part, and after extended periods that could drain the booster tank. However, it seems that a booster failure would do the opposite. A booster failure would cause a HARD pedal, not a soft pedal.

And thusly, I"m stumped a bit, lol. All of the things that make sense would cause a HARD pedal, not a soft one, it's really weird. That's why Master cylinder and ABS modulator are high on the list of potentials. But I"ll start with the cheap stuff first I guess, Brake fluid, then Master cylinder, then I'll do the front calipers because I need to anyways soon, then last resort will be the ABS modulator I guess.

 

not necessarily a hard pedal, ive driven cars with the brake booster off at work in the parking lot and the pedal doesnt feel hard, feels kinda soft and the pedal is way down low. pedal feels kinda soft without the booster and you have to mash it down to brake half decent

 

This happened to me and it turned out my new caliper was bad somehow i dunno if it was sticking in or out but sometimes the pedal dropped to the floor out of no where. I realized the caliper was the suspect because the rotor was rusting where the pad should have been in contact. So i warranty exchanged it and it never dropped to the floor like that again.

 

You can always rule out ABS by just removing the fuse for it and seeing if the problem is replicable immediately.

Not saying that's the issue, cause the brake booster idea posted above sounds like your particular issue, but just sayin'.

 

interesting insights on the brake booster and the caliper, thanks guys!

 

is the pedal soft first thing in the morning after sitting over night? seems like it would be but you didn't say. the problem seems like a slow leak causing loss of pressure which can be restored by pumping brakes, and that's probably why it doesn't happen in the city because you would constantly be applying brakes every few minutes keeping pressure up while on the highway there would be extended times without braking so pressure goes to almost nothing. so to me it seems like you might be pulling air like you thought, just very slowly so it appears to behave in the odd way you mention.

 

Bleed bleed bleed.

What causes a soft pedal? Air in the system.
Your fluid boiled, so there's air in there somewhere and it cant get out. It's probably a small amount, so the booster can overcome it sometimes, but other times it cant.

This is also the least expensive option to do before you start thinking about replacing boosters etc.

Once I bled my brakes because of sponginess after changing the front pads. I was convinced I could not have let air in the system. But, If I closed the doors had everything quiet, I could actuallly hear a very very small farting sound as I pushed the pedal! I kept bleeding for like 45 minutes after that until that sound disappeared.

The rock hard brake pedal eventually returned.

I have ABS, too, so there are lots of places for air to get stuck.

 

keep us posted on what you find.

 

Hmm good thinking Pilm! It's not soft in the morning, but this thought lends itself to the below idea as well



So it shall be done!! What a great explaination too, I wouldn't have thought this was possible but it does make sense that with more vacuum assist I don't notice the problem at all, doesn't mean it doesnt exist in the system still. I need to change the fluid anyways, so I'll do this first.

Also I'm pretty sure I can't boil brake fluid at auto-x, but who knows, I did drive this thing hard as heck!

 

Try spraying around the brake booster with some soapy water and see if any suds appear. That'll tell you if it's leaking or not.