# Strange brake pressure issue



## silentdub (Jul 24, 2020)

2013 Rogue.

Customer replaced the rear brake pads and rotors and then came up with a strange issue and asked me to take a look at it for them.

When driving, every once in a while say after about 20 stops the brake pedal will go to the floor acting as if there is air in the system, on the very next pedal pump, everything is fine, normal operation. This happens at high and low speeds, short and long distances. 

0 leaks, no wetness anywhere. Bled/Flushed the brake fluid, same issue. Perfect pedal, great feel, all wheels braking fine then on one press pedal to the floor, vehicle stops but in the same manner as if there was air in the system then again, very next pedal pump perfect braking.

My first thought is that maybe the master cylinder is damaged from when they pressed the piston on the calipers back in without removing the cap from the MC Reservoir.

Can anyone think of anything else to check for?

Maybe the brake change has nothing to do with it but scratching my head on this one.


Any feedback is appreciated.


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## rogoman (Dec 16, 2004)

There may be a problem with the brake booster. When the vacuum brake booster check valve is working correctly, applying pressure to the brake pedal is easy and very smooth. When the check valve is not working correctly, the operation of the brakes becomes much more difficult.


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## silentdub (Jul 24, 2020)

When the pedal is pressed slowly it sinks to the floor but when pressed hard or pumped it feels normal. Only when pressed slowly, eased on does it sink.


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## rogoman (Dec 16, 2004)

It's a symptom of air in the system.


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## silentdub (Jul 24, 2020)

Not really though. When you press hard it is stiff and normal. Air would give me a soft pedal all of the time.

when the pedal is pressed alowly I can actually see the fluid in the reservoir rise once stying depressed. Seems like one of the valves is shot in the MC.


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