Hissing noise inside vehicle when pressing brakes

[A1218]

1997 Nissan Pathfinder

I changed the passenger side front brake caliper today as the old one’s piston seized up and was dragging the brakes. I bled the brakes after and went for a drive, doing a few hard stops. It seemed a bit soft still so I bled it back again, but now I’m hearing hissing air like noises when pressing down the brake pedal. The noise is heard in the vehicle and continues to make it if I hold down on the pedal. I was not having this issue before…

Could something have failed from doing hard stops? I bled the caliper several times so I do not think it’s air. What’s strange is the air leak noise coming from pressing down the pedal and when holding it down it continues to travel further. The braking is okay, but not like what it was before and I can’t even engage the abs on an emergency stop now. If I pump the pedal with the engine off it gets very stiff, but no stiffness once the engine is on.

Could it be the brake booster check valve or the brake booster itself? Maybe the hoses for the brake booster, I have to check that but I’m skeptical it’s even that.

[VStar650CL]

Short answer, chances are your booster is shot. On a ride that old, the rubber diaphragm is probably degraded, causing it to tear when you pushed the pedal past its normal stop position during bleeding. That will cause the booster to suck in cabin air with audible whooshes or whistles under the dash. That's another reason I always recommend never pushing the pedal past halfway while bleeding (the primary reason being the risk of screwing up the seals in the Master). Too late for you, but maybe this will help someone else.

[VStar650CL]

PS - The gradually dropping pedal indicates the Master seals may be shot too,

[A1218]

Short answer, chances are your booster is shot. On a ride that old, the rubber diaphragm is probably degraded, causing it to tear when you pushed the pedal past its normal stop position during bleeding. That will cause the booster to suck in cabin air with audible whooshes or whistles under the dash. That's another reason I always recommend never pushing the pedal past halfway while bleeding (the primary reason being the risk of screwing up the seals in the Master). Too late for you, but maybe this will help someone else.
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[VStar650CL]

When the pedal keeps dropping to the floor like that, it generally means the Master is bypassing (fluid is leaking past the seals and cycling back into the reservoir instead of holding pressure). When it happens after a bleed, it's because a ring of crud accumulated at the normal stop position for the pedal and then the bleed pushed it past the crud line, causing the crud to score the seals. Stuff like that is always a risk when dealing with old rides. It sounds like you took precautions, but sometimes making one repair just shows you what else in the system was ready to fail. Anyway, yes, I'd recommend just replacing them both.

[A1218]

When the pedal keeps dropping to the floor like that, it generally means the Master is bypassing (fluid is leaking past the seals and cycling back into the reservoir instead of holding pressure). When it happens after a bleed, it's because a ring of crud accumulated at the normal stop position for the pedal and then the bleed pushed it past the crud line, causing the crud to score the seals. Stuff like that is always a risk when dealing with old rides. It sounds like you took precautions, but sometimes making one repair just shows you what else in the system was ready to fail. Anyway, yes, I'd recommend just replacing them both.

I guess better now than on the road one day. Looks like I have some work to do.

Is it something I can tackle myself?

[VStar650CL]

Been a long time since I did any brake work on an R50, but I don't recall anything funky about the job other than the inside studs being a PITA to access.

[A1218]

So I got a new booster and installed it last night. The only tricky part for me was wiggling it out the small area by the firewall. I bought a new check valve from Nissan and changed the hose as well. I now have a firm pedal again and the air leak noise is gone.

I actually left the master cylinder alone for now as it was changed several years ago (Nissan part), so it’s not old like the age of the vehicle at least. It also was not leaking into the old booster, so I guess all may be fine with that for the time being.

[VStar650CL]

[A1218]

One more thing, is it necessary to adjust the push rod when installing the brake booster? And can that be done from inside the vehicle by the pedal?

After driving it for a while this morning, the pedal feels firm when pressing down on it but it doesn’t feel as if the brakes engage right away. Also, the pedal appears higher than it was before. If I hold down on the pedal it doesn’t sink to indicate any sort of leak, the brakes just feel less grabby. Is it not engaging the master cylinder fully?

[VStar650CL]

Yah, that sounds like the rod needs adjustment. They generally do.

[A1218]

Yah, that sounds like the rod needs adjustment. They generally do.

I bled the system back and I can feel the brakes engage slightly if I lightly press the pedal. Much better than before and I can get the ABS to engage on the road. I saw a tool online used to measure the proper adjustment for the rod so I’m ordering it to double check, but based off current conditions, does it seem like the rod on the booster is pretty much correct?

Also, can an SAE J30r7 3/8 hose be used to replace the old vacuum hose to the booster or will this type of hose collapse?

[VStar650CL]

R7 hose is pretty much the OE standard for all underhood vacuum and low-pressure applications, so that should be fine. The idea with the rod is simply to make sure it's free at full pedal release. How far it travels before engagement is pretty much a matter of preference. I personally prefer 1/4~3/8" of switch flange travel before the pads engage, that way I can set my cruise switch a notch higher than my brake lamp switch and not flash the lamps at people when I tap the cruise off. That's just me, if you like instant response then set the free play very close.

[A1218]

R7 hose is pretty much the OE standard for all underhood vacuum and low-pressure applications, so that should be fine. The idea with the rod is simply to make sure it's free at full pedal release. How far it travels before engagement is pretty much a matter of preference. I personally prefer 1/4~3/8" of switch flange travel before the pads engage, that way I can set my cruise switch a notch higher than my brake lamp switch and not flash the lamps at people when I tap the cruise off. That's just me, if you like instant response then set the free play very close.

Is it possible to bleed the current master cylinder by along the same way as I bleed at the wheels? Basically having someone press down on the pedal and crack the lines to the master. I bled each point on the Pathfinder using the two person method and the order the FSM states, starting with the LSV, and about 10 times each spot until no bubbles seen in the bottle hose. The brakes are much better like I said but I still observe that I have to give it a pump to harden up more and get rid of that sponginess. Calipers are new, so are rear cylinders, and hoses are fine.

I will admit the master has run dry once after a brake line broke a few years back, but after changing the line, refilling, and bleeding, I did not really notice any issues. When I fill the reservoir I with brake fluid I notice bubbles or air lock occasionally that will make the brake fluid take some time to go down. The pedal holds firm pressure when the engine is off and you pump the pedal hard so I’m sure the master is not losing any pressure internally. So, can I give the master a bleed by cracking the two lines to it and pressing the pedal and will I have to re-bleed each wheel after?

[VStar650CL]

So, can I give the master a bleed by cracking the two lines to it and pressing the pedal and will I have to re-bleed each wheel after?

Yes and yes, in that order.

[A1218]

So, can I give the master a bleed by cracking the two lines to it and pressing the pedal and will I have to re-bleed each wheel after?

Yes and yes, in that order.

Is it possible to do just the master without having to redo the whole vehicle? Just asking because I’ve heard you can but if you cannot then I’ll just have to do the whole vehicle.

[VStar650CL]

Try it. Sometimes you can get away with it, other times you'll get bubbles.

[A1218]

Try it. Sometimes you can get away with it, other times you'll get bubbles.

Well, I tried it and it didn’t help, nor make it worse. The pedal simply travels too far and is squishy on first application. Pump it up when driving and it gets high.

This all started with a seized passenger side caliper piston that was holding the brakes. Changed the caliper and bled it, then squishy pedal problems started. The same day, the brake booster got damaged I believe from the pedal travel, so changing that rectified the air noise when pressing the pedal. There is no leak, the brake pedal gets hard and holds when the engine is off, start the car and it goes down like it should with the brake booster working.

I’ve bled from the LSV, down to wheels, and even the master. I’m stumped… I have a new brake hose to the line on the diff, as well as a new hose to the driver side caliper. The passenger side hose was fine, but this problem only started after that caliper was replaced. The rear drum brakes are adjusted, new hardware, shoes, and drums. But like I said only after the caliper change did all of this start.

Is it the caliper I changed , is the master, or can it even be the booster? I have no idea.

[VStar650CL]

It's pretty much impossible for a caliper to cause a squishy pedal, the only travel it performs is back and forth the width of the square-cut piston seal (the untwisting of the seal is what pulls it away from the pad). If you have drum rears then it's possible for the Proportioning or Metering valves to cause squish by putting too much pressure to the rears before allowing the fronts to engage. If it has one of those lever-type proportioners on the diff, that has to be bled before the wheels when bleeding the rears, otherwise it will never bleed completely out. If you have ABS then it's also possible (but unusual) to get air trapped in the internal reservoirs.

[A1218]

It's pretty much impossible for a caliper to cause a squishy pedal, the only travel it performs is back and forth the width of the square-cut piston seal (the untwisting of the seal is what pulls it away from the pad). If you have drum rears then it's possible for the Proportioning or Metering valves to cause squish by putting too much pressure to the rears before allowing the fronts to engage. If it has one of those lever-type proportioners on the diff, that has to be bled before the wheels when bleeding the rears, otherwise it will never bleed completely out. If you have ABS then it's also possible (but unusual) to get air trapped in the internal reservoirs.

Yeah it has the valve at the back, Load sensing valve, which I always bleed first. Then LR wheel, RR, FL, FR as per FSM.

Does it need to be pressure bled? It most certainly has to be air, pumping it when driving hardens it up. Is it possible for a master to be bad even though it holds pressure once the vehicle is off?

[A1218]

I was told by a Nissan tech to gravity bleed the brakes, running at least a quart of fluid through and that will probably get rid of the air.

[VStar650CL]

Worth a shot, or you might even consider a cheap vacuum bleeder. Sometimes bubbles trapped in odd places like the tops of thermal loops just don't want to move.

[A1218]

Worth a shot, or you might even consider a cheap vacuum bleeder. Sometimes bubbles trapped in odd places like the tops of thermal loops just don't want to move.

I’ll give it a shot. Quick question, is there a real reason as to why the manual states to disconnect the plugs to ABS system when bleeding? If the key is out the ignition why would that matter, or does it matter?

Does the ABS require a separate bleeding procedure?

[VStar650CL]

It doesn't matter, they just don't want people being stupid and trying to bleed with the key on. If you do, the ABS will try to re-pressurize its reservoirs and mayhem can result.

We use the Consult at the shop to bleed out using the internal pump, but you can crack the ABS outlet fittings just like the master to bleed it manually.