Loosing brakes when idling in traffic..

[Gerald]

Has anyone ever experienced this? My car has been doing this intermitantly for months now, but now it seems at any idle for a few minutes my car's brake pedal will become mushy so much it will almost touch the floor. Pumping the brakes will not bring it back either. The only way it comes back is go drive it for a couple minutes at highway speed or let it sit for 5-10 minutes..

Brake fluid? Master Cylinder? Any simple tests that I can perform?


Gerald

 

[99 R/T 10]

Hey Gerald,
I would guess you have a master cylinder problem. While sitting at a stop light your holding pressure to the brakes and the master cyl. piston may be allowing brake fluid to pass around the piston(allowing the petal to go to the floor). If you have to pump the brakes back up, IMO it is your master cyl. Now this is assuming you don't have a puddle of fluid at one of the wheels.

Mike

 

[Tom F&L GoR]

Since you can't pump it up, it could be the booster. We ran taxi field tests where the brake booster hose to the manifold would plug and the booster couldn't "refill" with vacuum quick enough. Maybe something with the brake booster check valve- distorts and won't seal when it's hot?

 

[Gerald]

Yes, I can't pump it up and there is no puddle anywhere under the car. Also, I JUST checked the fluid, it's COMPLETLY black. Could VERY old and VERY , VERY BLACK brake fluid cause any problems like this?

Gerald

 

[GTS Dean]

Gerald,

I sincerely hope that the black fluid isn't in your Viper! You should know better than that.

As for your pedal issues, I would guess that your power brake booster or the vacuum line from the manifold are giving you problems.

 

[Dixter]

Gerald.. Gerald.. Gerald..

Your fluid should be very much clear... Now you'll have to
bleed your entire brake system to see if its the fluid or
a different problem...

You might as well get a new set of EBC greens to go with the
fluid... Its going to take a little more effort getting your
fluid back to normal. Bleed and bleed and bleed some more,
until the fluid is perfect coming out of the bleeder valve's.

 

[Marv S]

So is that black fluid 10-50w or straight 30w?

 

[CoryMac]

If your car only does this when it's at idle or stopped, it's something wrong with the master cylinder 99% of the time. You could have air in your lines which would be compressed when you stepped on the brake and cause the pedal to go down further than normal, also.

 

[jrkermode]

I've heard of a similar problem from someone else with a high output motor. In that case I believe the problem was a case of cooking the master cylinder (bore expands too much, seals can't seal right and the heat damages the seals). I would look at ways to get more air circulating around the master cylinder (insulation won't help, it will only delay how long it takes to finally cook the thing).

The fluid change should also help. The black color of your fluid is a result of water in the system. When enough gets into the system the water vapor can cause a spongy pedal.