Thread: intermittent brake problem

Hi all,

now that I have the good stuff all fitted and the axle seal problem solved I started doing long and tough trips again. Since a few weeks though I have this weird intermittent break problem. I am not sure they are related but I have two seperate observations to share.

first of all, the electric brake booster takes a helluva long time to build up pressure every time I start the car, that is, when it's been standing still over the night for example. I replaced the accumulator on top of the booster last year so that is brand new. I know that the booster should kick in after every 4 times of braking. Yesterday I took a 150km long trip through some very tough terrain and on the way over there and back I started counting. Sometimes the booster is heard when I'm just driving along, sometimes it's 3 times, sometimes it's the expected 4.

Second, when braking under certain circumstances it seems like if the front right wheel locks up for a fraction of a second. When driving high speeds on the motorway and braking gently to slow down for an exit or whatever, the front right brake makes the kind of popping sound and feel you would get when you drive a car that has been stationary for months and the disc brakes have all rusted up. It's a fairly short but very noticeable thud accompanied with a yerk to the right ofcourse since that wheel is doing a lot of braking. I can also feel it in the brake pedal. The ABS does not kick in, the light does not come on and the wheel does not lock up (no burning rubber is heard, seen or smelled).

Yesterday when coming home from my trip I noticed the braking situation got worse, at one point a took an exit and during braking I had the opposite happen; the brake "faded" for a fraction of a second and during that the brake pedal did not kick up or harden but sank in a few cm's further. As did the sh... in my pants I can tell you!

Once back on the straight, with fairly slow speed I tried braking and it was like I only had 1" of disc brake, with what seemed like the interval of the turning wheel I had short moments of heavy braking. Not sure of what to make of that and the prospect of beeing stranded in the middle of a sunday night on a deserted road I decided to keep on driving. On such an empty road I would not need to touch the brakes voor 60km at least and at least then I would de close enough to home that I could walk if needed. When I got close to home I engine braked all the way down to 40kph and ever so gently applied the brakes, on hand on the handbrake, prepared for the worst: nothing. Nada.

The last corners to my house nothing happened and the range braked like a champ as it should.

I'm pretty sure the problem is not gone so I am at a bit of a loss to start searching for this problem. The discs, pads and calipers have all been replaced recently. I followed the bleeding instructions to the letter and the brakes are nice and firm as they should when they work. The only thing that I can replace are the caliper brackets and those pins over which they slide. I did take them out, cleaned them up and greased them when I replaced the calipers. They were getting rusty but they moved in and out smoothly.

I remember reading about some ring in the ABS modulator failing somewhere, it is made of plastic and it did something catastrophic failure, but I can not remember where and I have been unable to re-google it nor am I sure that it is related.

So, anyone have any ideas?

Thanks!
-P

Originally Posted by Drano

Thanks Scouse for the link back to that earlier article about the brake modulator. A very important bit of servicing advice!

I will get mine checked out - every press of the brakes seems to have a different amount of travel for the pedal - maybe it's the booster playing up in mine?

These beasts do not have a vacuum booster, they have an hydraulic brake pump with a high pressure accumulator.

I'd start with the modulator, there is also a timing test you can do on the brake pump to see if it is in spec and if the pump is running a lot then the accumulator may well be suspect.

Steve