reflexuk wrote
When we tested the handbrake it would not lock the wheels and would currently fail an MOT!
I knew there was something wrong......
He seems to think most caliper handbrakes operate a cam an when the lobe wears it can no longer apply adequate force. Can anyone on here confirm or deny if that's how our rear caliper works?
That is how the Leon's rear calipers work, but unless you are in the habit of handbraking at every corner, the mechanism doesn't get enough usage to wear significantly.
As has beeen said already, the MOT does not require the handbrake to be able to lock the rear wheels on the brake testing machine, only to meet a minimum efficiency, which is 1/3 of the efficiency of the footbrake.
Just to confirm your earlier messages, none of these calipers is new? So you've replaced your calipers with second-hand ones from somewhere.
Why did you replace the old calipers? Had something gone wrong?
Are the new-fit calipers identical to the old ones? If not it's possible that the handbrake cable fittings aren't compatible or need adjustment of some kind.
It is compulsory for the handbrake to operate on a completely separate mechanism from the footbrake. So you can't, for instance, have a handbrake lever that operates a hydraulic piston feeding into the rear brake lines.
The mechanism used on our cars is a mechanical linkage from the handbrake lever to the rear caliper pistons. At the caliper, the handbrake cable operates a lever that acts on a rod attached to the piston inside the caliper - you aren't going to get a lot of leverage this way, and certainly not nearly as much as you get from the hydraulic system, with its servo-assistance. There are various ways that the force is transmitted to the piston, usually involving a cam of some sort, sometimes a lever.
Inside the caliper, the handbrake rod has a screw thread and is screwed into the piston. The caliper is so arranged that, with the handbrake off (i.e. in normal driving), every time you apply the brakes the piston is pulled out of the rod, unscrewing itself a little bit. The rod has a stop so that a small clearance is maintained when the footbrake is released, and thus the handbrake clearance is automatically adjusted every time you apply the footbrake.
Now, these principals have been used on rear brake calipers for donkeys years, and most of them are pretty reliable. But the handbrake mechanism can still jam, usually by the outside bits getting corroded or the seals where the motion gets transferred into the hydraulic system (to get at the piston) leak or get jammed with dirt.
If your handbrake is non-MOT compliant according to your friendly tester you need to find out why. Is it just one side that's bad, or both? How many clicks of the handbrake lever are you using to get full application? Are the handbrake cables adjusted properly at the caliper end? Lots of things could be wrong, especially if your newly fitted calipers are not the same as the original ones. However, I'd be looking for jammed pistons, not auto-adjusting as expected and so leaving the handbrake lever unable to get them to contact the disks. This is more likely if the new (old) calipers had been left drained of fluid, in a scrapyard for instance.
Just for completeness, other (more expensive) cars with rear disk brakes have different mechanisms for dealing with the handbrake
problem. Some have completely separate handbrake calipers and pads: these can now be hydraulic as they are separate from the main system. Another way is to have a small set of drum brake shoes acting on a small drum cast into the center of the disk - in many ways this is the best , as drums can be made self-servoing, acting to increase the braking force with rotation. All the extra components mean that these methods are not applied to most "family" cars.
It could be worse. Some Citroens had handbrakes acting on the front wheels. I've never investigated how or why, I don't have a C&G and ten years in the plumbing trade necessary to the study of Citroen hydro-mechanics
