Q: Brake pedal sinks all the way to floorboard

asked by on July 29, 2016

While pulling a load, the brake pedal began to sink all the way to the floor. I replaced the front calipers, master cylinder, and new pads all around. Also, several times I bled the system. When the car is off, the pedal feels very firm. When I start my car at idle, I have a good feel in the brake pedal, but when I begin to drive a couple of feet and then stop, the pedal sinks almost to the floorboard. A boost test was done and it was good. My car previously had a 2007 Is2 corvette v8 swap, no ABS, and has braided brake lines as an upgrade. It also has disc brakes all around. Would a bad check valve be the cause of the brake pedal problem or do you think the motor is causing too much vacuum?

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Hi. Sounds like a sweet setup you have there for your 1998 Nissan 240SX. It sounds to me like the lack of brake pedal pressure could be due to a leaking brake booster possibly or a brake booster hose collapsing under vacuum. I would suggest that you have the brake booster smoke tested for leaks to monitor the brake booster hose while the brake pedal is operating to verify that it keeps its proper shape without collapsing under pressure. Have a certified technician inspect your brake system to let you know exactly what repairs should be done.

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