While the 75 was stored in my garage the reservoir of the rear master cylinder disintegrated leaking brake fluid all over the place and giving the RHS footplate a severe case of Japanese frostbite. The replacement m/c reservoir should reach me tomorrow, but I wanted to check before filling up with brake fluid that the caliper pistons were working OK. Curiously the pistons were in the pushed fully into the caliper position and I was trying to think of a way to see if they would move freely. I'd read in a post on this forum that it works to use an air line to drive a piston out of its bore in the caliper. That prompted me to think of using my car tyre inflater which is is driven via a 12V power socket.



It worked very well and on a setting of approx 3.5bar the pump applied enough force to the disc pads for me to be satisfied that one or both pistons are not frozen. I had already checked to see if any brake fluid had leaked past the caliper seals, but fortunately it hadn't.

Chris

Having the caliper pistons pushed fully home means they should be ok as they would have been kept lubricated by the fluid and hopefully protected from corrosion. If you get a problem you are more likely to find the brakes binding so just keep an eye on the, if they bind they will get hot.

I had a similar problem with the rear master cylinder but a seal kit was used to repair rather than replace it. It has lasted fine.

92KK K100LT 193214 wrote: ...I had a similar problem with the rear master cylinder but a seal kit was used to repair rather than replace it. It has lasted fine.

I dismantled the rear m/c and there was some corrosion, but the seals looked OK and the corrosion was above the normal travel movement of the piston seal, so I decided to polish the bore instead of buying a seal kit. I should know today or tomorrow whether I made the right decision.

Thanks for your comments. Chris

good stuff chris ...

Just to round off the saga of the rear m/c: the replacement reservoir arrived on Monday and I fitted it and filled up with brake fluid yesterday. Haven't been able to road test the brake yet, but the brake line works fine. Regarding the screw adjuster at the front of the m/c, how do you judge how far to screw it in? Should there be a tiny bit of free play before the screw rod engages the 'pusher' cup that bears on the end of the piston? Thanks in advance.

Chris

C_Mick wrote:Should there be a tiny bit of free play before the screw rod engages the 'pusher' cup that bears on the end of the piston?

That`s correct...and this small amount of free play is quite important....to be shure that the piston returns fully to its rest position....every time.

To avoid a binding rear brake, which can be hotter than h...