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1980 Honda CB750F front brakes drag?

First I‘ve had the bike for two years and don‘t know when the brake fluid has been changed (so easily two plus years)Here are the symptoms,At night in 55deg temperature, front brakes operate fine.During the day at 80deg temperature, front brakes clamp up rotors, bike will move, so its not completely seized. Popped the master cylinder reservoir and noticed water on the other side of diaphragm. Even when brakes drag, the front brake handle still operates properly.My theory. the brake fluid is so old and combined with water that during the day, the water steams up creating air pressure and clamping down on the brakes.Should I just get the brake fluid flushed or should the master cylinder be rebuilt? Your thoughts? Thanks.


Incorrect theory: Brake use creates more heat than the difference between day and night will. The brake system needs a rebuild, from one end to the other. All the seals in the calipers should be replaced. I would recommend getting braided lines. There are no longer braided brake line kits available for this model, so you need to find a shop that will make them up for you, using line and banjo fittings. I performed this on my 1981 750F many years ago, and got greatly improved braking performance. However, compared to my newer bikes, front brake performance is weak and without feel: Scary! Brakes have improved a lot (and bikes have become much lighter) in 30 years.
May 28, 2018
ok buddy the brake fluid will def need a change every yr is preferable check the brake hoses for porousness then take the calliper off the disc check the dustseals are not broken the water could have affected the metal on the cylinder if it has a white dust will be evident electrolitic action between the brake piston and the cylinder wall if it is so take them both to a dealer and get them cleaned and overhauled to be honest that sounds to me what has happened the electrolyte is stopping the piston returning fully into the cylinder of the unit,brakes are damn important when ya need them, ya do, and they have to work or ya gonna be a satistic if ya not so handy on the tools just change the fluid of that fails take the whole system off the bike and to a honda dealer let him sort it he knows what he is about if ya feel capable however get a haynes manual and do it yourself it will be much cheaper up to you which hope that has helped. ride free
May 28, 2018
A complete overhaul of the brake lines is in order. The system should be flushed every year if you're a purist, not many of us are (we'd rather be riding). That CB's system is old, expect to have some expenses involved. The master cylinder should be cleaned, for sure. You may be able to just replace the brake fluid if money is tight, that is better than nothing.
May 28, 2018

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