Strange Brake Issue - losing pedal pressure but only when the engine is running

[blaketjones]

Hi All,

I'm fairly new to NSX ownership (month two now) but have already spent a bit of time with my head in the forums and workshop manuals thanks to a few common problems (failing window regulators, hot A/C) that my relatively low-km 1992 NA1 decided to surprise me with! Thankfully most of those were easily remedied... but I'm quite literally stuck on this one. I'll describe some symptoms, to see if any of you doctors can help me diagnose exactly what's going on. I live in Tokyo and my Japanese isn't amazing so I don't want to go straight to a dealer without really understanding what the issue is.

• First, started to notice infrequent hesitation at idle. Thought maybe it was a dodgy spark plug. Occasionally car will splutter on start.
• Second, dashboard throws a few lights at me on a long drive - the brake light, ALB and TCS lights (although my memory is not perfect).
• I arrive after midnight from my drive and car goes into storage until the following Saturday morning, when I have time to take it out and see what's going on.
• At this point, notice that the brake pedal basically goes to the floor - leaving me with a minuscule amount of braking ability. Brake warning light still illuminated, but ALB and TCS lights are not.
• Notice that once the engine is switched off, brake pedal pressure returns. Odd. Play around a bit, stupidly press the soggy pedal many times.
• Inspect the reservoirs. I cannot recall to what level they were last filled, but top reservoir (master cylinder) is basically bone dry and bottom reservoir (ABS) is slightly over-full.
• Inspect vacuum hoses in the frunk, can't find anything broken or detached. Have not been able to inspect any vacuum lines in the engine room or under the car.
• Inspect for leaks, but find nothing. Look all around master cylinder, ABS unit, all four calipers and no sign of brake fluid. Unfortunately my car is in 'machine parking' (remember what Bow Wow's car is in in Tokyo Drift?) so although there is a brown stain under the car, I have no way of knowing if it came from my car.

I have a feeling it might be vacuum related which may explain the engine hesitation at idle, and the loss of pressure when the engine is running(?). But, what's the deal with the empty master cylinder, but no obvious leak? Perhaps a seal within the ABS unit has perished, causing a transfer of fluid to the ABS reservoir (I did read of that happening to someone here).

Thanks in advance for any advice.


P.S. sorry to anyone who posted in my welcome thread - I'm unable to respond yet as I'm a 'new user' and nobody will approve my comments...

 

[Larry Bastanza]

You are confusing me with some of your comments:

You mention "Master Cylinder" which would be clutch, not brake. But are you saying your brake reservoir is empty? There are two reservoir on top, large one is brake, small one is clutch. If it is your brake reservoir, time for a brake cylinder replacement. What is probaly happening if this is correct is when the engine is running and the vacuum is there for power assist it will put more pressure on the brake cylinder and make a poor brake cylinder fail, vs. engine off.

Also if it is your brake reservoir that is empty, time to look at ALL the calipers to see if you have a leak at a wheel location.

HTH,
LarryB

 

[syndicate]

Have you looked inside the car where the brake master cylinder goes through the fire wall? Your fluid loss could be inside the car.


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[Old Guy]

Do as Larry B. advises and check the calipers for leakage. But first, top up the master cylinder reservoir, mark the level with tape and then pump the brakes repeatedly. If you have leakage having lots of fluid in the reservoir should provide visible evidence of the leak location and the level dropping in the reservoir provides double confirmation of a leak. If no signs of leakage at the calipers then check for leakage where the master cylinder mounts on the brake servo unit. Particularly along the bottom of the mounting surface. If you have confirmed that you are losing fluid and cannot find visible signs of external leakage, time to pull the MC off of the booster and check. I had an acquaintance who had disappearing brake fluid with no external evidence of leakage (not on an NSX). The brake fluid was leaking out the back master cylinder piston seal and filling up the brake booster with brake fluid. Not sure that is possible on an NSX.

Edit: your brake warning indicator light staying on is an indicator of low fluid level in the reservoir. If it goes out when you fill up the reservoir, don't take that as an indication that things are OK.

Syndicate's comments about fluid leaking into the cabin are correct for a leaky clutch MC. I am not so sure about a leaky brake MC. I think the seals on the brake servo would have to have failed for the fluid to get out the back of the MC, through the servo and into the car interior. If the brake servo seals had failed you would definitely know that.

 

[syndicate]

That's true.


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[asylum]

I think that your brake master cylinder failed. You do not always see the leak when it happens. Top up with brake fluid, brake and look if it empties. If you do not see leaks elsewhere (brake lines, brake calipers, ...), change the master cylinder.

For the hesitation, I would change the spark plugs anyway (not expensive and easy to do) and, by the time, check for humidity and/or rust on coilpacks. Comeback if the problem is not solved.

 

[blaketjones]

Quick update - today a friend brought some tools and aluminium jack around. As per asylum's recommendation, I topped up the brake fluid, then pumped the brakes for a few minutes to look for a leak. It took a while, but it seems that there's a very small leak in the front left brake line, which is (with power assistance from the booster) causing the fluid to aerosolozie and coat the suspension with a fine mist of brake fluid.

The car is now at the nearest Honda dealer (cool story - the tech who is looking after me worked on the original production line installing driver-side door trims) getting all four lines replaced, and perhaps a once-over if they have time before the week of national holidays coming up.

Thanks to all of you who provided insight and helped work through a list of possible failures - in the end it was something quite simple.

Hopefully the next update is a short and easy one!