Brake problem - warped disk?

[Graham Jung]

The car's a Superlight with the standard Superlight brakes that's done about 17,000 road and track miles. Recently I am having to pump the brake pedal (only once) in order to get full pressure and hence the normal amount of travel. This happens on road and track regardless of how hard I've been using the brakes. My first thought was to bleed the system and replace the fluid, but I've now done this twice and the factory have had a go, but it hasn't cured the problem. There is no obvious vibration under braking, and although I haven't used a dial test indicator, there does not seem (by eye) to be much run-out on the disks.

 

Any ideas to save me the cost of a new set of disks welcome

[Alex Wong1697456877]

I can't see why this should be a fault in the disks. If you're happy that the system is air free and the fluid is new, I'd be looking at the master cylinder next.

[Morls]

I've had a similar problem on a Metro engined Metro, This was traced to very loose wheel bearings allowing the discs to wobble between the pads and push them back.

 

Discovered this going down a bumpy hill. Eeeek.

 

The seven's wheel bearings are ever so easy to check, there should be about 1/16" of play at the wheel rim according to Tony Weale's book.

 

Mark

[Grim Reaper]

Yes,sounds like disc movement is causing the pads to be pushed back into the calipers.

If you have access to a dial gauge,test the run out of each disc to see if its warped,if not then bearings are the next step (unless you have floating discs a la Motorcycle)

Does the brake padal go down slowly if you keep pressure on it? May be a seal leaking if the other two things don't show anything wrong.

[DanB]

I had this problem on the Furrybird after a little off-road incident loosened off the front wheel bearings. If you pump up the pedal and then leave the car stationary over night, does the pedal stay firm? If the pedal only goes soft when the car's moving then the most likely explanation is that the pads are getting knocked back. Either due to excessive run-out on the discs or loose wheel bearings.

 

Dan

Furrybird Q660 KKL