Front (disc) brake sticking on

[robint]

Presume this is a corrosion or seal issue in the caliper - brake grips fine (but rather - no make that very - wooden feel) but doesn't release properly - eventually easing off after a mile or two. Any obvious problems to be aware of or other possible causes??

Bike has done about 20k miles in 33 years and is on 2nd set of pads and second disc - a tom cat and its marking scent destroyed the first one over a winter storage !!

[EVguru]

The front brake feeling 'wooden' is down to the mastercylinder being the wrong size. Like many 1970's disc brake setups, the bore is too big. The same wheel, disc and caliper is used on the 250 twin and feels fine, so fitting the smaller bore 250 mastercylinder is a good fix. New, they're something like £60 from NLM. The original cylinder is just right with twin discs.

A sticking caliper is probably down to corrosion at the bottom of the seal groove. Contrary to much teaching and some text books, the piston does not move in the seal, other than to take up wear. Instead the seal 'rolls' into the base of the groove, which is cut at an angle of about 10 degrees. When the seal relaxes, it pulls the piston back.

A while back I fixed a Z400 front brake for someone. The corrosion under the seal was so bad I could barely pump the piton out. I cleaned it up using a dremmel and a nylon brush wheel. The seals were unavailable and the original looked horrible. All sorts of dents had been made in the outside edge by the pressure of the corrosion, but after being left in brake fluid for a couple of weeks the seal regained most of its original shape and was refitted and gave good service for a couple of years until the bike was sold.

I DO NOT RECOMMEND DOING THIS! If new seals are available, USE THEM!

[Brubru]

It could also come from the piston of the master cylinder not returning to its normal position. But in this case you should notice an unusual free play in the brake lever.

I am currently having this problem with the rear brake of my 500; cleaning the caliper did not help and I discovered only later that it came from the master cylinder. I'm currently trying to clean the master cylinder (with a toothbrush...the Dremel would still be too big )

Some people kindly suggested to stop braking, but I'm not sure it is the best solution, even in Cadwell...

Bruno

[robint]

Removed the caliper and after a little wriggling managed to pop the pistons out fairly easily - after having worked out how to clamp them in place (pipe pliers) so that I could get both out in turn. Looked very clean, so gave them a wipe, made sure the sides of the caliper where the pads sit were clean - bit of dirt there - refitted and bled the brake and seems fine.

Less wooden - will cope with the problem the mileage I do - but had to pop out for the paper. So 3 hours and 120+ miles later in the sunshine got home and started to read it. Miss M ran perfectly, brakes better than I recall and roads free of traffic - nice .