91 NSX brake poblem ... frustrated to the max

[bobbytinvab]

all,

I usually can find my answers by searching the forum, but I can't seem to find a thread that describes my problem.

I have owned my car since 95 and have got the most annoying of brake problems. I have been dealing with it for a loooong time and am truly frustrated. The car has all the symptoms of a bad master cylinder. I have changed it (a couple of times) most recently about a month ago, changed out all the fluid and bled the brakes multiple times to get the air out. I've bled brakes on a number of cars and realize that the NSX is a bit difficult, but all was good when I got done. I then drove the car 250 miles from my farm back to the beach with no problems. I put in the garage (have not moved it) and now less than a month later, the brake pedal goes to floor as if there is air in the system. I guarantee there will be air in there ... this has happened to me before! Master cylinder level is at the full mark and there are no visible leaks anywhere. Where the heck is the air coming from?

I've searched and read up on the ABS flush etc, but the ABS system seems to be working ... the pump will cycle for less than 60 seconds and shuts off as expected. No ABS light etc except for initial check on start.

I am truly at a loss as to what is causing this. If it's the ABS, then I'm considering going to the newer design from Science of Speed since the initial design has supposedly been problematic. Especially considering the cost to rebuild the older design.

thoughts anyone?

Bob

 

[Hapa88]

How often does the ABS system cycle for less than 60 seconds? Everytime you turn on the car?

 

[bobbytinvab]

no not every time, only after it's been sitting for awhile (weeks). If I drive it the next day for example it doesn't cycle.

 

[warrenw]

You say the pedal goes to the floor like there is air in the system.
Does it go to the floor with a spongy feeling, or does it drop to the floor with steady pressure, can you pump the pressure up with multiple pumps, or does it just drop to the floor like when bleeding the system?
The ABS should not cause this as the two brake fluids do not normally mix.
If it is just air in the system since your master replacement. have you tried a vacuum bleed? using something like a mightvac?
http://www.mityvac.com/pages/products_bcbe.asp

 

[Wavey1]

What Warren said, or a pressure bleed with a Motive pump (if you can get a cap that works).

 

[bobbytinvab]

Thanks for the replies ... I've been offline for a bit.

To answer your questions:

The brake pedal goes to the floor with little pressure ... it will pump up and hold that pressure ... but loses the pressure when I release the pedal and depress it again.

I've gravity bled, used a mighty vac as well as the phoenix reverse flow system previously ... each time everything is good to go after multiple bleeds. Drive it for a day or two ... everything seems fine. It sits for a couple of weeks and I'm back to the same issue.

This last time I bled the master prior to install, then used the phoenix reverse flow to get the air out lines/calipers and then let it gravity bleed to settle it out. Pushed a lot of air out with the phoenix system, burped some fluid with the gravity bleed, then closed bleeders and put on the master cylinder cap ... drove the car. I'm thinking I finally got it ... not so fast ... dang it.

I realize that the ABS and regular brake systems don't normally mix, but I was looking at the ABS "modulator" diagram in the tech manual and there does seem to be a place where air may reside and could somehow get into the regular system. Maybe I'll try to bypass the ABS system electrically and hydraulically as a test, to see if that eliminates the problem before buying anything new. I have a bunch of other "old" cars and never have encountered this type of problem before. I just want to fix the NSX ... It's a great car.

thanks for reading,
Bob

 

[NSXPR]

Same happened to me months ago. All I did was changing the Master Cylinder and now it's breaking like a charm.

 

[dquarasr2]

I assume you traced the brake lines all the way from master through proportioning value and ABS lines, all the way along the hard steel lines, all the way through to the calipers. If you have not done so, try that and see if there is weeping anywhere along those paths. Also, ensure that every single solitary connection is dry.

And lastly, and I have the most confidence in this being the root cause, do you have speed bleeders? If so, remove them and replace with standard bleed screws.

One question: when you are getting air when you bleed, is it always the same corner or is it all four? That can help you track down where there is a breach in the system.

Don't discount NSXPR's comment. There is no guarantee that the new master cylinders you installed are without problems, although that might be a longshot.

Good luck, sorry this post is a bit late but I didn't see the original thread until just now.

P.S. Hey, I missed noticing that five posts ago I reached the 1,000 post mark!