MOT fail- brake efficiency 45%???

Just had my car back from the MOT centre and was expecting it to sail through as it had its annual inspection service a week ago. But got a call telling me it had failed on brake efficiency. The front brakes were only 45% efficient and the rears were just ok, but they told me these should also be better. The discs are fine as far as I can tell, they were all replaced 12k ago. The ABS works ok when you really stand on them, but I have to admit i have never really felt like they are doing a particularly good job. The MOT centre had no suggestions about what might be the cause and I have not yet had a chance to speak to the indy garage that serviced it as they are closed Saturdays. Any ideas what might be the route of te problem?

My brakes are not exactly great. Discovered that the inside of the discs were quite rusty(quite common). It was suggested I brake hard to get the rust off. When you brake do you get a rubbing sound just before you stop?

Had a similar thing with my MOT last year, as Brendan said the inner faces of the discs rust and that was exactly my problem, also the calipers had partially seized. Not sure if the 996 has the same disc shields as the 993 but they make inspection of the inner faces really hard!

Just took a couple of pics of the inside, although a bit pitted surely nothing that would make the brake efficiency to 45%? And if they were bad presumably this should have been picked up at the service?

a bit of hard braking should sort that out or you could get them skimmed http://www.skimmydiscs.co.uk/find_a_lathe.html when was brake fluid last changed?

The thing is though, if the ABS cuts in then the brakes are as effective as regards ultimate stopping power as you could ever expect them to be. (Abs only cuts in as the wheel is about to lock up which is the point where stopping power is at max) So how do they come by the figure 45%? Are they out of balance? Is the pedal pressure too high? - Servo not working? Maybe the tester was not applying sufficient pedal pressure? (Porsche cars generally need a firmer push on the pedal!) Might be a good idea to ask them for more detail!

The brake fluid was changed a year ago, but it is difficult for me to judge the braking against what is usual for a 911 as this is the only one i have driven. I appreciate the pedal needs pushing firmly to get any real effect but does feel a little lack luster. It is weird that the abs comes on, which as stated indicates the car is braking to its full potential. will keep you posted as to the findings next week.

I have not MOT'd my 911 yet but on my other cars I remember that the ignition is off during the brake test in order to disable any ABS function that can screw up the real efficiency readings. I'm not sure how they calculate the efficiency but I guess it is something like the ratio of the weight on the wheel versus the braking force. I don't think it is anything to do with pedal effort - since this is not measured. The pics of the inner disc surface don't look great to me and suggest perhaps glazed pads - which will certainly give poor results and may not be easily rectified some simple hard braking when driving. The only time I have failed an MOT on this is on my old 19 year old MX-5 - it was simply and cheaply fixed by new pads.

Woodie it does look like the pads are not evenly applying pressure to the disc, mine were partially seized and my discs looked the same but on the outer part only. The pitting/corrosion on the face is similar to what mine looked like, heres a pic! Even hard braking wont clean of the disc face if they look like this! BTW mine PASSED an MOT and a 12K service looking like this! Skimming is a good idea, check they will be in spec after, but new discs are pretty cheap and your going to have to change the pads anyway. Mark Edited for my spelling...again

As a forerunner to the garage looking at the brakes i decided to do brake off against a friends new Scirroco, while not being that scientific it would at least give me a bench mark. The first thing we had to do was find a suitable piece of road. We plumped for a side by side test, to save any potential embarrassing mishaps. Finding the road i one thing waiting for the right traffic is another. Anyway after a few aborted attempts we counted down over the hands free and slambed the anchors on. Both cars stopped in a near identical distance, certainly not what i would expect for a car that had just failed its MOT with a 45% braking efficiency. So i dropped her off this morning to my local Indy who was surprised with the fail and suspected idiocy at the test centre. They performed another MOT and yes a pass with 78%. They said that it was possible the figures were incorrectly entered on the machine, but it should have been flagged. Well a black mark on Nationwide MOT Centres and a +1 for my local porsche specialist. I must remember the MOT is due at service time next year, so I can get it all done at once and save myself a load of grief.

I would be looking at the pistons in the calipers for seizure.This will cause uneven distribution of brake force. You can do this by removing the caliper off the disc and with an assistant brake against a mock disc(something of the same thickness) observing that all the pistons are moving wedge a pair and pump the pedal push the movers back and block them and repeat untill all of them are moving. If you have some sticking and they free off the caliper should be overhauled. The easiest piston will move move first you have restrain them to get the rest to move. No need to start the car.

Woodie sounds like you got a good result in the end! Not sure if the 996 calipers are the same as the 993's? The 993 has steel guide/liner plates, the aluminium caliper can corrode behind the plates and cause them to "swell" out very slightly, this can cause the pads to stick. Its an easy(ish!) fix.