My 2000 A4 has a weird problem. The brakes fail once and only once in the morning, every morning. The pedal drops completely to the floor. The brakes come back within a few seconds. This does not occur during the day -- unless the car has been sitting for at least 6 hours. Then again.

My mechanic is baffled.

Help please.

 

Time to find a new mechanic! Does the car ever lose brake fluid? Has your mechanic flushed the brake fluid out and bleed them? Air in the lines? Vacuum problem?

 

Thanks for your response. The car is not loosing brake fluid. Brake performance is fine during the day: the pedal is neither spongy (suggesting an air bubble) nor hard (suggesting poor vaccum). Nevertheless, something occurs overnight, so that upon first starting, I have ot wait about a minute before the brakes become useful.

If the vaccum one-way valve were failing, the brakes would be hard in the morning until the vacuum rebuilt. This is not the case -- as the brakes are spongy for a brief period in the morning. If the car sits for 6-8 hours, they'll be spongy briefly again.

My mechanic has proposed alternatively bleeding the lines and/or replacing the master cylinder and/or replacing the power booster and/or replacing the brake lines.

I want my brakes fixed -- but am not too thrilled about a fishing exercise as I replace the entire brake system at high cost to me.

Have you experienced a similar problem? What area of the brake system would you rate as likely having the problem.

Thanks!

 

I'd do the least expensive thing first. Bleed the lines.

 

I too would start by bleeding. When was the last time the brake fluid was replaced?

 

I had a similar problem on a chevy I had it turned out to be the master cylinder leaking internaly when the car was cold it was at its worse... Because the piston was at its smallest then. It let the fluid slip by the piston

When it got worse any time you pushed the brake hard the piston would loose its seal and the pedal would hit the floor

Get a new mechanic.

 

Again, thanks for the responses.

The brake fluid was replaced in February of this year.

SlimChriz: I'll try pushing the brakes in hard to replicate your experience with your Chevy. Although, I'm not looking forward to having my brakes hit the floor -- it'll accurately diagnose the problem.

If that is not the case, I will have my mechanic start out by bleeding the lines. If that doesn't work, then I'll have him aim for the master cylinder. I'll update this thread with my results.

 

check ur brake fluid or brake lines, either that get a new mechanic.

 

Hey!

I recently got a 98 A4 1.8T M/T and I found that several days after I bought it, the Brake pedal would just go mysteriously to the floor and lose all resistance randomly. After the 2nd time this happened I brought my car to a mechanic.

At first I thought it was just the pads. My rear brake pads were worn so I had them replaced. However, when he checked out the brake system, he found that someone stupid had used DOT3 brakefluid instead of DOT4... and this melted the rubber in the Master Cylinder. The Brake Reservoir tank also got filled with melted rubber and had to be replaced.

Its a costly repair, about S$240 for everything, but its worth the peace of mind. Good luck.