Idle drops when pressing on brakes.

[bobokiller]

Hey guys, I've got a 93 NA causing some trouble.

I changed out the front brake pads about a month ago. Anyways, Today as I was driving I noticed whenever I would press on the brakes, a red light on the dash would come on. I pulled over, and as I was pulled over and while the car was on, I was pumping the brakes watching this light turn on and off and I noticed that the brake pedal was never buiding pressure, just felt spongey. Along with the brakes not building pressure, it would cause my car to drop idle to around 400-500 and almost die. When the car is off and I pump the pedal, it feels completely 100% normal, yet when I turn the car on, i get that sponge feeling again. Anyone have any ideas how I should approach this? Could it be a vac leak in the brake booster? Is it gonna be a costly fix? I really don't have much money right now as I'm a broke college student so hopefully its something I can get sorted out myself. I'd hate to end up having to sell this lovely car.

Thanks to anyone to helps.

[ThisIsSparTTa]

Warning lights coming on when you brake is a sign of your alternator about to crap out. I might have made it ~15 minutes of driving after mine started doing it. Mine started with dash lights, then the radio started cutting off, then other accessories. It finally just died, and that was that.

After that I would follow the FSM for testing your brake booster, but that wouldn't cause lights to come on.

[bobokiller]

Alternator was changed a year ago. I don't think it's a part of the issue but i'll keep that in mind. I really have no explanation for the red light. It only came on when I would brake, i've never even seen that light before so I don't even know what it's for. possibly a warning light for a rear taillight, but I don't know. As I was driving though, I hit a bump and the light never came back on. I was able to find some people with similar issues as mine on different cars and i've seen them say replace the check valve for the brake booster, so i'll give that a try.

[ThisIsSparTTa]

I would test the valve first. Also, aftermarket or oem alternator? A year is about right for some aftermarkets.

[Ziggy1621]

Is it the brake light on the dash coming on? If so, it is telling you that you probably have a light out. The spongy brake pedal usually means you need to rebuild the master cylinder. The rebuild kit isn't expensive and is easy to do

[ThisIsSparTTa]

If you go the MBC route, I got a Centric 17/16" from RockAuto for like $88 shipped (it's almost double that at CZP and Z1). Rebuild kit is $65+shipping. Totally worth the little extra to not have to deal with the rebuild and also know its been properly assembled.

On a side note...if anyone needs a OEM 17/16" MBC to rebuild...I have a core for you for cheap...

[Ziggy1621]

That is a good price, but OEM is better quality, which is why the cost. I guess it depends on what your plan on doing with your car

[ThisIsSparTTa]

True, but the fact that Z1 and CZP both stock it means they are giving it some kind of endorsement for being legit. I'd definitely take that over whatever random crap that auto store is pushing. Also, I thought Centric made some of the Z32 OEM MBC's, or maybe for other Nissans. Too lazy too look it up atm.

[Ziggy1621]

I wouldn't say they make it legit.... They both also sell AMS products

Now, not that I'm saying Centric isn't good, I use their pads and rotors, but for the master cylinder, I'd recommend the rebuild

[Metacom]

The brake light will flip on as others have said if your 3rd brake light (AKA stop light) bulb is out and you press the brake pedal, or the soldered connections in the brake light switch (behind driver side brake light) have broken. Another cause for the brake light to flip on is low brake fluid in the master cylinder.

You may attempt to bleed the brakes to improve pedal feel before replacing the master cylinder, if there is a leak in the lines somewhere though this wont do much.

RPM dropping when your depress the brakes, happens to my Z too. Since your alt was replaced a year ago, are you sure it produces the proper amps? How is your battery/battery connections? Its common among older cars, old connectors, etc.