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.
Agree with the lack of free play. At the first service, the bike shop adjusted mine as they reckoned there was too much free play - on the ride home after just a few miles the bike was getting slower until I stopped at a roundabout and it wouldn't move. The rear wheel was locked solid, brake disc was glowing orange and the pads were on fire and that was despite not having even used the brake. Leave some free play in the pedal.
The brake is shite at the best of times - never used it at all during my ownership of 748/916*. When you do want it to work, it doesn’t….when you don’t want it to work, it does! *ditto for 749, M1100 and 1098.
lots (an understatement) on this subject on this Forum over the years, but search engine sabotages easy retrieval.
You absolutely sure your aren't touching the rear brake lever. And make sure the reservoir isn't past the full mark when cold
As others have indicated, there is a tiny (0.6mm) compensating port in the master cylinder that must be uncovered / unblocked by the piston when the pedal is released. Three issues can cause this not to happen: 1. Lack of freeplay stop the piston travelling back sufficiently. 2. The port is plugged with shit especially if it's not been serviced / fluids changed in a long tme, and the oem rubber hoses can deteriorate with time. 3. The master cylinder piston / liner interface can become sticky. Dunno why, and it's rare but sometimes the plastic liner interface clearance to the piston gets reduced such that the spring has insufficient force to return the piston fully so the port remans covered even with sufficient freeplay. In this instance, given the bike has not moved for 20 years - I'd be replacing everything or at least stripping and servicing checking everything and replacing the hose with a good braided one.
Thanks everyone for all this priceless information. This is stuff which can't be found in Haynes or similar; the alternative is to learn the hard way! I wasn't even aware that the pushrod is adjustable! I had a bike in the distant past where I needed to grind off some of the rod's length but can't remember which bike or why I did it! But yes, the recurring symptoms are exactly as described by Dukedesmo above. When I acquired the bike the brake and clutch fluids seemed to be very clear and pale, suggesting that they'd been changed prior to its sale. It did (somehow) pass its MOT last October though it hadn't been ridden as the mileage has been unchanged for about 20 years. So I'm wondering if the pushrod had been tampered with to get the bike through the test. The brake's fine when its cold so nothing would have shown up during the test. I'm gradually wading through stuff to do so a new master cylinder and hose is added to the list. The front brakes are great so I'll adjust the rod for now and replace the cylinder etc as part of the rolling restoration. The front will provide 95% of the stopping power so I'm happy to live with a less than ideal rear for now, so long as it'll allow the rear wheel to turn! Just back from the Newark autojumble today, so it'll be back to the spanners tomorrow. Again, many thanks for all the info.
Sorry, ducv2, but yes, I'm absolutely certain that my foot isn't touching the brake pedal. On the last run I put the toes of my boot on the footpeg to be certain that I didn't touch the brake. It heats up with just the wheel turning and absolutely no use of the brake, which is why I initially wondered if the old pads had swelled. I reached the point where I'd pretty much eliminated everything else. But having seen all the replies I'm sure it'll be the push rod.
If you get stuck with the brake locked due to the pushrod length you can simply remove the pushrod and the pressure will be released - it is held in by a clip. Obviously you'll have no rear brake if you do this (who uses it anyway?) but better than calling for breakdown.
