Brake pedal hard to press when engine is under acceleration

[pyrophilus]

I drove my '05 LE from NYC to St. Louis this past week.

I do all of my own repairs (replaced front struts, rear shocks and coilsprings), replaced all fluids before the trip, and brake pads/discs looked fine.

Halfway along the trip, I accelerated (WOT) to overtake a slow 18-wheeler. as I was passing the cab, there was a cop on the side of the road. I immediately let off on the gas and stomped the brake to find that the pedal was incredibly impossible to press down. I panicked (thought I lost brakes).

After 1/2 to 1 second, I ragained the brake pedal.

While driving through the mountainous roads of PA, I would accelerate uphill, and then when a car jumps into my lane, I would try to slow down, and find that the pedal is again hard to press down (for a second).

While I was in St. Louis, I had no choice but to bring it to local shop, and they said that the master cylinder failed test (I think they were BS'ing me). I had to pay $475 to get it replaced, and on the trip back, noticed the same thing.

I then tested it to find the exact symptoms:

The pedal goes completely stiff when the engine is straining. If I am cruising at 60mph, and then press the accelerator 1/2 way down and after car starts to take off, then at the same time try to press down on the brake (while foot is still on gas), I get the hard brake pedal forever, until I let go of the gas, or if the car goes to steady cruising.

I have also tried 3rd gear to rev the engine high, and as long as I am not touching the gas, high-rpm alone does't make the brakes act up.

So it seems that my pathfinder gets hard brake pedal while engine is straining under load... The pedal feels just like when you turn of the engine and then pump the pedal with engine off until all vaccuum is gone.

The only place I found that has exact same problem is a ford super-duty diesel forum but them I think diesels do not have same brake system that we do...

I have searched the internet for two hours and couldn't find anything except the ford pick up forum and a VW bug forum where a cracked vacuum line from engine to air filter box OR vacuum line to brake booster was the culprit.

Does anyone have any clues on what could be causing my problem?

To complicate things even further, I paid for a pdf FSM for my pathfinder years ago. I put the pdf files on my home NAS RAID server, and the file server DIED. If anyone has pages of the FSM that tells you the steps to accessing the vac lines on my '05 I would also be greatful

-Sage

[disallow]

Could be a worn master cylinder, or possibly a vacuum issue, as you've mentioned.

Master Cylinder available on Rockauto for $60-100, so thats pretty reasonable. Its not a hard repair at all, and a good opportunity to change your brake fluid. Hardest part is bleeding air out of the system.

The FSM is available on this forum at:

http://www.thenissanpath.com/viewtopic.php?t=2

Good luck! Keep us posted...

t

[pyrophilus]

Could be a worn master cylinder, or possibly a vacuum issue, as you've mentioned.

Good luck! Keep us posted...

t

I had a local shop replace the master cylinder while in St. Louis, so the prob wasn't the master cylinder. Considering that the brake acts like it lost vacuum during WOT, I am hoping and praying that it is the vac hose from engine to booster. I just pulled the hose, and while it does not look like it has cracks, the booster end does feel "harder".

I just looked at the diagram on courtesy Nissan, and the plastic piece with barb that at the booster (that the vac hose goes into) is called "check valve", which in my book means it's a ONE-WAY valve for gases???

I am hoping that it is either the hose, or the check valve (or both)...

[disallow]

I had a local shop replace the master cylinder while in St. Louis, so the prob wasn't the master cylinder.

Ahh I missed that. I guess I assumed that you didn't get it done, it sounded pretty unreasonable...

Check out the brake booster diagnostic on page BR-18 in the manual I sent you the link for...

[jspitz]

Sounds like it may be the same as my (assumed) vacuum leak. Is the pedal hard to press on the first push after the car is shut off? I am just starting to notice that my brake pedal is harder to push under load, and actually can feel significant differences in pressure if I rev the engine even a little bit while stopped.

My original post was here: http://www.thenissanpath.com/viewtopic.php?t=7735

Please let me know what you find

[smj999smj]

Usually when the diaphragm inside the booster fails, you'll hear the "hiss" from under the dash. Yes, the check valve is "one-way" and I would definately take a look at that and the vacuum hose.

[jspitz]

Sounds like it may be the same as my (assumed) vacuum leak. Is the pedal hard to press on the first push after the car is shut off? I am just starting to notice that my brake pedal is harder to push under load, and actually can feel significant differences in pressure if I rev the engine even a little bit while stopped.

My original post was here: http://www.thenissanpath.com/viewtopic.php?t=7735

Please let me know what you find

UPDATE: My issue was the Brake Booster Check Valve. Hoses all looked OK, so I checked the valve which I could easily blow air through in both directions. Generic versions were about $10-30, the Nissan part was about $4 with tax, but had to be ordered. To my surprise it actually came with a new grommet. Installation took about 5 minutes and requires no tools, but pliers would make pinching the hose clamp a bit easier.

Hopefully your issue is this simple and cheap as well.

[pyrophilus]

Yup, my problem was also with the check valve. I spent the $4 and got it from courtesy.

The valve wasn't checking anything in any direction.

It should allow air passage from booster to engine direction, but not from engine to booster...

Mine was allowing air to move in both directions but it was actually harder to push air from booster to engine, opposite of what it was supposed to do.

I replaced the vacuum hose, and the check valve, and now the brake feels fine, no more hard pedal, except when push gas to floor and then try to stomp on the brake pedal, it initially does press and slow the car down, but I notice that it loses power gradually (a vacuum leak somewhere else?).

But in sum, I didn't realize how much power the brakes lost from the failing check valve. in the past, if I try simulated panic stop from 5mph, the car would attempt to lock wheels and engage abs, but on dry pavement, it would simply "slow" down. Now with the new check valve, when I stomp on pedal at 5mph, you would be thrown into the front windshield if you didn't have seatbelts.

I actually would recommend that everyone check their check valves (no pun intended)...

[volvite]

Pictures of the culprit area and bad part?

[smj999smj]

Refer to "47478" at the following link:

http://www.nissanpartszone.com/Page_Pro ... ntsIndex=6

[volvite]

Thanks

[Rotorian]

Refer to "47478" at the following link:

http://www.nissanpartszone.com/Page_Pro ... ntsIndex=6

Thank you.