Brakes Efficiency Percentages

Last week my '71 Early Bay van failed her MOT on a couple of points (both now rectified and retested - so she's through), one of which was that the parking break efficiency was only 13%. The tester gave me a report showing the figure above, which also showed:

Front Service Brake 38%
Rear Service Brake 27%
Total Service Brake 66%

I was curious about these figures and meant to ask when I went back, but forgot.  Since then I've done a little research, and here's what I found out:

Apparently the percentages are calculated by dividing the total weight of the vehicle (with everything in it) by the force required to stop it (they have a machine to test this at the test centre) and multiplying by 100%.  A little searching on the web revealed that the service brake needs to be at least 50% and parking brake at least 16% for an MOT pass.  However this left me wondering: how much better could they be?

What I found suggested that it's possible to get significantly higher percentages and, surprisingly, to even exceed 100% mark.  Sounds crazy that something can be more than 100% efficient doesn't it?  But consider the way the percentage is calculated and what would happen if you put the brakes from HGV on a moped.  :-)

Unsurprisingly such high efficiencies are not recommended and according to herbie275h on the VZi forum:

normally the limit is reached by the tyres which skid around 85-90% of the vehicle weight

Various Internet sources also suggested that 60-70% is about normal and this sounds quite reasonable to me after finding the following titbit of information that SpunkMoney posted on the motester.co.uk forum:

The Highway Code stopping distances ("A car in good condition, with good brakes in good conditions blah-de-blah-de-blah....") base their actual stopping distance (ie: NOT the reaction part) on a deceleration of 0.66g - that's 66% braking efficiency.

So I guess 70%-ish is what we ought to be aiming for and be happy with.  I guess it also makes sense that the individual front and rear efficiencies will be less, and that the handbrake (which according to herbi275h needs to be enough to hold the vehicle on a 1:6 hill), can be even less than those and still be acceptable.

Thanks also to Samba63 on VZi who pointed me in the direction of SpudMonkey's info.  ;-)