97 240sx brakes go out after driving for a while

[SlidnSideways]

The problem just started this past thursday (10/2). I have a 45 minute commute to and from work and my brake pedal slowly got easier to push as time went on. By the time I got to work the pedal would go straight to the floor not stopping me at all until I pumped the brake then pressure would return for a min or two. I checked out my brake fluid and it was a bit low so I filled it up. I didnt have time to test it out until after work. It seemed to work fine. Yesterday it did the same thing though. After checking my fluid again, it doesnt seem like im losing any brake fluid anywhere as the master cylinder was still full. There also seems to be a burning smell that smells similar to a clutch burning. Any ideas?

ThanksTim

[carkook]

Im no mechanic but I am 4 weeks (of 6) through my brakes class at school so Ill see if I can help you out.

Well in a brake system fluid should never dissapear. It can get low in the master/cyl from worn pads, taking more fluid from the mast/cyl to replace in the caliper. The only thing I can think of is that there is air somewhere in the system, or a leak. The leak could be somewhere where it splis onto something hot and getting burned, and I have also heard of the fluid going back in through the brake booster and into the engine and getting burned. Either way you need to go over the master cylinder and check for leaks and follow the brake lines all the way to the wheels to insure there are no leaks and bleed the system.

[silviaS13milknhoney]

sounds like a heat related brake fade. Check to see if your pads or rotors are glazed or if there are any "hard spots" in the rotor. Try taking the pads off and using 180 grid sandpaper take the first layer of pad off. And yes fluid can go back into the booster, rare, but can happen. Also check the master cylinder for operation, while someone steps on the brakes look for fluid movement (should bubble up) in the res. Other than that, carkook takes care of it.

[carkook]

It does sound like brake fade, but the fact that the pedal goes to the floor means there is another problem. Brake fade occurs when the rotor/pads get too hot but would not be the reason for the pedal going down to the floor. Has to be a leak or air bubble. Check it out let us know wats up

[SlidnSideways]

Thank you guys for your responses. Ive parked the car until the weekend where I should have enough time to try out your suggestions.

If there was a leak though, wouldnt I be going through brake fluid? The master cylinder looks to be holding the fluid pretty well. Also, Is there a reason why the pedal would gradually get softer to push until it eventually goes to the floor after about 45 mins of driving?

Again, thanks for the help and ill let ya know what I find out this weekend when I follow up on your suggestions.

ThanksTim

[carkook]

If its holding the fluid then it probably isnt leaking. There has to be air in it somewhere but why it goes to the floor after driving for a while instead of all the time im not sure.

[silviaS13milknhoney]

Although if there was air in the system the problem would appear all the time. I'm racking my brain on this one, wish i was there to check it out. Anywho keep us updated im curious on this one.

[maj Andres]

Bad master cylinder, maybe? take it out and bench bleed it to see if it's pushing out the fluid. Then reinstall it and bleed your brakes. Inspect those caliper pistons while you have your wheels off too.

[carkook]

NEW EVERYTHING FTW

[silviaS13milknhoney]

maan yeah dude just drinks some berr. no big deal man who needs brakes

[carkook]

yeah downshift instead

[slow40]

had a brake fluid leak and didn't know about it until I was running my buddy's stealth, came up on traffic and only the pass rear worked, locked it up and smoke went everywhere, scared the crap out of me. oh yeah, if it does leak any brake fluid it will eat the paint off, so best bet is to get it fixed.

[carkook]

Hes not losing any brake fluid so it isnt possible that he has a leak. I was also thinking that maybe the fluid is contaminated with another fluid that reacts to heat.

[silviaS13milknhoney]

Yeah maybe the brakes overheated causing the brake fluid to lose some of its viscosity. I don't know why it would happen only after 45 min of driving, and seriously thats one hell of a test drive to make sure things are working properly. Well things that he could rule out are:1. Air in the system2. A leak3.Bad metering valve or proportioning valve4. Brake lines5. ????

[SlidnSideways]

Ok so I finally got a chance to look at the car. I decided to start with the rear driver side wheel first for a few reasons. I inspected the fronts not to long ago and when I had smelled the burning smell it seemed to be coming from that wheel.

My pads were dangerously low... almost to the point where there was no pad. Is it normal for pads to wear down differently like that? My fronts were replaced at the same time as my rears but are almost brand new looking while my rears were damn near down to metal.

Also, the boot for the piston on my rear driver side wheel had been all chewed up and there is quite a build up inside the piston area. Ive replaced the pads but the boot is a problem. I have no clue where to get one. Local parts stores all have to order these as part of caliper kits and the wait on them is alittle over a week.

Anyway my brakes are still going to the floor but I havnt bled the system yet either. When I do this and replace the boot ill let ya know how it worked.

Thanks again to all who helped!Tim