The rear brakes on a 916 are as simple as they come, right? No ABS, no servo, in fact, just a master cylinder, a slave cylinder and a rubber pipe between the two. So why do I have the following problem? I acquired the bike towards the end of last year. In the previous 20 or so years it hadn't turned a wheel. I took the bike out for its first run last week. Two problems; the clutch slave cylinder developed a leak and the rear brake seized on. New clutch cylinder and good low mileage rear brake caliper obtained. Clutch solved and replacement rear caliper now fitted. So off I go again, confident that the two issues have been solved. But after around two miles or so, the rear brake seized on, exactly as before, despite the replacement caliper. The rear disc is immensely hot when seized but frees off again once it's cooled down. Today I made a point of not using the rear brake and my right foot was kept well clear of the pedal. The brake discs on the SP were originally cast iron and these ones are possibly the ones which came with the bike. They're a bit rusty in the non-contact areas but are straight and worth keeping. The pads are possibly 20 years-old though not particularly worn so I'd popped them back in. Is it possible maybe that the (probably old) pads are swelling with just minimal contact with the rear disc? I'll obviously replace the pads but if they are swelling then it's a new one on me! I've discounted a sticky master cylinder as I didn't even touch the pedal on the last run. Any help with this will be very much appreciated.
Sounds like the rear brake pedal/actuator rod is not correctly adjusted. You need to free off the actuator rod that goes into the master cylinder so that there is a bit more clearance before it actuates. Otherwise, when the system gets hot the brake will be applied as there’s insufficient clearance. The brake light switch screw should also be checked as this can also contribute.
Two possibilities with rear brake. If the caliper is know to be good, then it’s the master. You’re not getting full release of fluid pressure by the sounds of it. First check there is some free play in the lever. You adjust the pushrod to obtain this. You want about 5mm of pedal travel before the pushrod contacts the piston in the cylinder. Second is the master is knackered. After being laid up that long I wouldn’t be surprised. They’re available new and not much relatively. Don’t bother trying to rebuild. It’s too important.
Very many thanks for your replies. There is some free play at the pedal when the bike's cold but maybe not as much as 5mm. Of course everything's solid when it's seized with no play whatsoever. And that's without using the rear brake at all. Push rod seems to be the first port of call so I'll back that off and see how it goes. Probably replacing the master cylinder is good advice too. Front brake fortunately seems to be exceptionally strong. Thanks again for the sound advice.
What state is the hose in? My 916 sat for a number of years before I got it and the hoses were blocked up. Compared to what the master can produce there isn't much force returning the fluid to the reservoir.
The hose looks to be OK but if I'm doing the master cylinder I'll replace the hose too. Have had another look at the bike since receiving the above replies and I reckon the pedal's too tight. Reckon the main problem's the push rod adjustment, as advised. There's some replacing of bits to be done and was intending to gradually work through the bike whilst making the most of this weather. Have spent lots of time and dosh recommissioning the bike over the winter and all seems to be working well, except for the rear brake. But in reality, replacement of the master cylinder and hose is a pretty quick and relatively cheap fix. Thanks everyone for the help provided.
