Need some help from the experts:

'83 K100 8v, no ABS. Never had rear brakes. replaced the brake disk (Motobins), the brake pads and the master cylinder (second hand unit off a K75). The master cylinder has good pressure and there is no flow back under pressure that I can see. No corrosion in the cylinder bore. I also replaced the bleed nipple.

Bleeding these brakes are a pain! Eventually, I used a big syringe attached to the bleed nipple to suck the brake fluid through the system till it was clear and without air.

Brakes worked well. Did not lock up under hard breaking, but very effective.

Next morning: No brakes, no pressure, nothing. No leaks and no flow back as far as I can see.

Please help

I was cleaning the underneath of the bike last weekend and found something that may be of interest. Its only one of a possible number of issues.

I noticed the gaiter on the rear brake master cylinder was a bit distorted and had come off. I decided to investigate as I had found been finding a not good rear brake. Pulled the gaiter to one side and saw that the pedal was not acting on the piston. There is a plastic plug which has a recess on it for the brake pedal push point and this sits into a recess on top of the piston. Or should sit in because it wasn't. All the brake pedal was doing was pushing the plastic plug to one side but having nil effect on the brakes. I reset it all and put it back together and hey presto a working rear brake. It's an easy check but fixing it does probably mean pulling off the brake pedal.

In my case it happened recently when I changed the rear brake light switch.

Sounds like the second bleed hole in the bore of the MC is blocked. looking in the open end there will be one hole about 2-3mm in diameter and a much smaller hole directly behind it. That small hole is probably blocked so that when you let the plunger return to the rest position fluid isn't drawn into the MC to replace what has been bled out. It is not easy to see or clean out. I have used a bristle from a wire brush held in a pair of long nose pliers to get to it clear.

Front or rear? Olaf has assumed rear, Rick seems to have assumed front (unless I've mis-read him). I read it as rear, because you mention replacing a disc (singular).

Dai wrote:Front or rear? Olaf has assumed rear, Rick seems to have assumed front (unless I've mis-read him). I read it as rear, because you mention replacing a disc (singular).

Just goes to prove there's always two sides to every argument.....even better both might be correct Korrect.

It is the rear brakes.

I should have read the title... sorry to both you and Rick. To add to Rick's comment, you will need to dismantle the rear brake assembly so that the master cylinder is on the bench. To see the two holes I'm pretty sure you'll need to remove the inlet pipe on top. Be sure to get a new one (and its O-ring) before starting because you can almost guarantee that it will break during removal.

Thanks for the advice. Will check it out. In bed with the flu and its damn cold down here. Brakes will have to wait a while.

Dai wrote:I should have read the title... sorry to both you and Rick. To add to Rick's comment, you will need to dismantle the rear brake assembly so that the master cylinder is on the bench. To see the two holes I'm pretty sure you'll need to remove the inlet pipe on top. Be sure to get a new one (and its O-ring) before starting because you can almost guarantee that it will break during removal.

Do you know where I can get one of these inlet pipes?  It isn't shown on any parts lists I have seen. 

I had my master cylinder sleeved last year and the shop damaged the inlet pipe when they removed it.

It is almost impossible to get the pipe out without breaking it. I believe Mazda have a suitable replacement in their parts list but as yet have been unable to find it. I was told this by a person I met that used to work for a brake resleeving place, he said it had the Mazda logo but he only knew that much because his boss treated it like a state secret.

Brembo. It's common across a lot of their motorcycle master cylinder range. The problem is that you'll need to locate a genuine Brembo supplier and talk to them because the company don't deal direct.

Motorworks (www.motorworks.co.uk) at a £10 each. I just called them to ask about it - the part isn't listed on the website.