Hi,

I've been lurking for a while fixing some issues with my daughter's 1992 240. Great info and forum! Now though, I have a real problem and couldn't find a good answer searching. The other night while driving up a steep hill, the car completely lost it's braking. The "brake failure" light came on, and the pedal seemed to be normal (all this is according to my 17y/o daughter) in that it didn't go to the floor, but there was no braking even with strong pressure on the pedal. She got it to the side of the road and turned it off while waiting for help. We got there, started the car and the brakes seem to work fine. Not too spongy, quick stopping, no real pulling. I brought it in and looked for leaks. I did find a bit of weeping on the right rear caliper and will change it out. The reservoir was down to exactly the "min" level mark. No other leaks in any lines or at the "proportional" valve.

Any ideas?? Could the steep hill have triggered the brake failure light from a low reservoir? Could that cause the brakes to not work? Why would it start working again on the same hill? The car does have ABS but she didn't say she felt the pedal pulsing. If the proportional valve failed, would the pedal have pressure but stop at the valve?

I can't let her drive it until obviously feel it's safe again (and now she's driving my car!!!)

Thanks for any input!
.
Evan

 

Welcome.

That is strange. Maybe since it was so low, that you got some air in there or something, and that might have trigger the light.
How are the brakes wearing? That has never happen to me before Mine doesn't have ABS though.

Wait and see what (Moderater) tech thinks.

 

I would inspect the main booster vacuum line and see what it looks like. Also check the check valve and make sure all of that is good.

I wonder of the hose sucked shut and then when she hit the brakes she wasn't poushing hard enough since there was no assist.

 

The brake fluid should be DOT 4 and changed out within 2 years. It has happened to me, old DOT 3 overheating on hills and faded brakes. Not to the floor, just like the brakes went away. Replaced the fluid with fresh DOT 4 and the brakes firmed up.

 

Hey Guys,

Thanks for the input. The brakes seem to be wearing fine and even. The pads are all fairly new. I checked the vacuum line and check valve, both look good and it held vacuum from last night. I did find though that I'm pretty sure I topped off the reservoir last month with some DOT3 fluid. Kicking myself for not checking the manual. Given that, what problems would arise from mixing them and would it actually cause an immediate failure, and then work fine again? Once I get the system back together I'll have it power flushed and changed.

For now, this is how I'm trying to figure it out. She said the pedal felt like normal, just not stopping the car. I wish she really stomped on it to see if it was just the assist that wasn't functioning.

Air in the system?? The hill was steep, but not crazy steep and the level was at, not below the min. line. Wouldn't it still be spongy with that much air in the system?

Master Cyl.?? The pedal would have gone soft to the floor and probably not come back working fine ten minutes later.

ABS?? maybe, she didn't say it pulsed at her, but she was probably in panic mode. ABS light did not come on either, just "brake failure" light.

Proportioning valve?? Don't know anything about this. Is it capable of failing like that and then working again??

Fluid?? Maybe. She wasn't doing any heavy driving and the hill was steep but short uphill. She went through a few turns and then noticed the failure light after the last turn. She was going @ 10 mph at the time. Could the fluid break down that fast and then come back fine with a little cooling??

Which components of the brake system could actually fail suddenly like that and then turn back "on" again?

This is quite the mystery! Unfortunately I'm not finding anything conclusive and won't really feel too good putting her behind the wheel again until I do.

Thanks again!

 

I would flush the system and see from there. My guess is something with the assist system.

 

Thanks Tech,

Yeah, that'll probably be my course. I'm figuring it's in the assist as well, but I'm not sure how it could fail like that. I'll post again if I find something different.

Thanks!!

 

Update... finally got back to fixin' the car. New E-brake cable installed and e-brakes adjusted. New caliper, rotors and pads and now bleeding the system through to run out the DOT 3 fluid I added. My question is: Anyone know the brake system capacity? Or at least give me an idea how much fluid/reservoir refills it should take to flush out the system? I had time to kill waiting to get back to working on the car so I opened the rear bleeders and let the system drain out over time. I then today refilled the reservoir and started with the RR and pumped almost a full reserv. to fill the line. Then went over to the LR and bled that through several cycles. I then went back and rechecked the RR and it's bleeding well still. All said and done between the two rears I pushed through about 1 1/2 reservoirs. My help just left so I can't do the fronts til he gets back so I thought I'd get some input from you guys. My other question is for bleeding the fronts. Do I now go to RF upper then lower, and then to LF uppper and lower or should I do the two circuits separately? The second reservoir has not moved in level, do I even need to bleed that circuit (I'm missing a lower bleeding screw on the LF, someone put in an allen screw)? When I added the DOT 3, it just topped off the large reserv. and was weeping out the RR caliper.

Thoughts??

 

I would try bleeding the fronts just for the heck of it.
It is hard to say how much it will go through. I just bleed till all the air is out then maybe one or two more times per caliper to make sure.

If you decide to do the fronts start with the RF caliper then do the left front.

 

Thanks Tech, How about the order of upper/lower? RF upper then lower, then LF upper then lower? Right? I figure another reservoir per side should flush out the old fluid, do you think that'll be enough?

 

That will work I have tried upper first and lower first it really never made a difference for me.

 

mine did the same thing it was two small leaks in the gaskets in the master cylinder