3 replies to this topic

[Millsy]

Hi,

I'm very new to mini ownership having only bought my first mini a few weeks ago.

I've found that when I first drive my '98 MPI Cooper that the brake pedal needs pressing to the floor to enable the brakes but after 20 or 30 minutes the brake pedal can only be pressed about 25% of the way and the car doesn't coast as well as it did. It feels like the brake pedal is pressed down when it isn't. There is also a very noticeable loss of power, which I'm thinking is due to the brakes being applied (even though they aren't).

I apologise for the lack of information, but like I said, I'm very new to the mini scene and very very new to anything mechanical. If you need any more information on the problem, please just ask, but I would be very grateful for any advice or guidance on this.


Thanks

[Ethel]

Feel the front wheels or brakes (carefully!), if they're binding they'll be hot.

The caliper pistons could be seized so that brakes bind, and expand with the heat. When they've had a chance to cool down the clearance increases as the pads have worn as a result of the binding. As soon as they get warm again, they seize and start the same cycle.

[AzMaN]

as above and in my previous experience with binding brakes they also have a distinctive smell usualy but that depends on how much they are binding.

another method of checking, try pushing the car when its cold on a flat surface, then again when its hot from the same position, see if its harder to move. Ethel's method is easyer although remember brakes will be slightly warm after use in normal conditions, and that temp can range from how hard you drive.

Edited by AzMaN, 28 April 2012 - 05:40 AM.

[tiger99]

This is, with at least 90% probability, the classic symptom of the master cylinder piston not returning fully, so hydraulic pressure can't be fully released. There have been a number of people lately chasing round all sorts of other imaginary problems, with brakes or clutch, when that was the real problem. Don't drive the car like that, as well as brakes binding, they can fail totally due to pad wear because the fluid in the system can't be replenished.

First check that there is a pedal return spring fitted, and in good condition, and the pedal shaft is moving freely. Yes, I know pedal box access is for contortionists, but it has to be done. Taking the seats out and putting a good cushion on the floor helps. Undo the split pin, take out the clevis pin, and pull the master cylinder pushrod down fully. Check that it stays fully down when the pedal is reconnected. If not, find out what is preventing the pedal returning fully, it may be the brake light switch, which may need adjustment.

All of that assumes that you don't have a direct acting servo. If you do, the principle is the same, the master cylinder piston must always return fully, but the details are very different. I am not familiar with the details of their mechanism, but the same fix may apply. But you also need to be sure that the servo is not sticking, which will obviously cause the same problem.

The fix, if there is nothing evidently broken or sticking, is to introduce shims between the master cylinder and its mounting (or you could in theory get a very slightly shorted master cylinder push rod). Item 20, part NAM1718, on the Somerford web site, link below. (Check that the gasket is fitted, as not having the thickness of that could be sufficient to cause the problem.) You use these in pairs (they are split so you can fit them by pulling the master cylinder up, without needing to undo the clevis pin), and ideally use a blob of silicone where they meet, and lightly on both faces, to prevent fumes leaking into the car. Obviously you can make these yourself if you have the facilities. I would cut a piece of metal to the size of the master cylinder base, drill all 3 holes, then chop it in half. Ali is ok.

You might actually want to try shimming before having to take the clevis pin out!

http://www.somerford...page=page&id=57