Brake Squeal, Disc Brakes
Brake squeal is a common complaint among vintage motorcycle riders, more so than among riders of more modern machines. Part of this is due to older technology, and part is due to differences in how we use brakes today.
Brake squeal, whether disc or drum, is generally a matter of vibration. That is, the friction pad or shoe, or in a few cases the disc itself, oscillates -- just a fancy way to say vibrates -- at a very high frequency, creating the squealing sound. Automotive-originated fixes such as high temperature grease applied to the backs of the pads serve to damp the vibration but not remove it and can be very effective, but tend not to be in motorcycle application with its smaller parts and likely different vibration spectrum. One fix that was developd by Honda and used for a sporadic period in the early 1970s was a takeoff on the goop idea, actually a gasket that was added behind the brake pad. This gasket was a standard part on very early CB750 fours. I have replicated this solution on a number of bikes, Hondas and others, with gret success. The gasket must be hard and thin, not typical squishy gasket material as that will negatively affect lever feel. I used to use gloss coated license plate shop logo placards. These were thin, tough, and relatively waterproof, and lasted 10 years at a stretch. These placards are hard to find now, but cereal box cardbaord is about the same thickness, though not as hard and not surface treated, so can affect lever feel slightly and do not last as long. Simply trace around both pads onto the cardboard, cut out, punch the requisite retaining pin holes, and insert. Against the backs of the pads, of course!
Modern brake calipers have anti-squeal springs, oddly shaped steel clips that bear against the brake pad, preloading it and preventing it from wiggling very much. Make sure these springs are in place and correctly installed. They mostly work, but the gaskets are still needed on many of these calipers.
A few motorcycles have gaskets under their brake discs which act as resonance-damping parts. Make sure when servicing brake discs (getting your wheels polished or powder coated) that these special gaskets are not left out. They are very useful for other brake issues too, as we will explore in a minute. Another way manufacturers have attacked the squeal problem at the disc, notably Harley-Davidson, was through a spring installed around the outer circumference of the rear discs of certain models, Shovelheads for example. This spring served much the same purpose as the leather strap machinists install temporarily around a brake drum while turning it to reduce noise during the operation.
Brake Squeal, Drum Brakes
Speaking of brake drums, they have their own set of problems that result in unwanted noises. The rear drums on many 1970s Japanese bikes have shoes that can vibrate just as disc pads do. One solution is to install an o-ring on the single leading shoe brake shoe pivot just above the shoes. I have seen this effectively damp oscillation of the shoes, and stop squeal. But be careful to not interpose this o-ring between the shoes and the pivot, for reasons that should be obvious and have to do with your safety. Honda also produced a number of drum rear brake systems in which the shoe return springs contacted parts of the drum casting, promoting a harmonic amplifying situation. Careful relieving of this area of contact has produced worthwhile results. Another little-known Honda peculiarity is the rubber washer that is supposed to be on the anchor strap bolt. Its purpose is to dampen noise.
Brake Disc Pulsation
Brake disc pulsation is very common on old and new bikes. There are actually three types of brake disc pulsation, something that is known only to professional technicians. That is, warpage of the disc is only one of the causes, though on vintage machines probably the most common cause. Disc warpage of course simply means the disc is no longer flat. The new floating rivet mounting systems used on today's bikes are supposed to help this, not so much by preventing warpage as by allowing it to occur temporarily, then recovery is supposed to happen and the disc return to straight again. Doesn't always work of course, and today's minimalist discs probably don't help either, their mass being so little as to be easily overcome with heat. It is possbile to correct a warped brake disc by having it ground. Yes, ground, not machined. The process is one advanced shops practice, and is the only reliable way. The best technique grinds the disc on both sides at once, thereby ensuring even thickness all the way around the disc. Older brake discs are thicker and alllow more room for this type of repair, whereas newer ones are so thin they often cannot be resurfaced, and they may even be stamped with a warning to that effect. In the case that resurfacing (grinding) either isn't an option, didn't help or whatever, there is something you can do. On certain Honda and Kawasaki models, there are gaskets under the brake discs. These gaskets allow a certain "give" in brake disc attachment bolt torque that you can use in your favor. By noting where the high spot is on a brake disc, you can then slightly overtorque the attachment bolts (within reason) near this point, and "tune" out muh of the perceived warpage. Far from a hack way of doing things, this is atually a Honda school taught procedure. If you have two discs on the wheel, after noting each discs high spots, remove the discs and rephase them, high matching low and low matching high. Naturally, this wont remove the disc warpage, but it does something seemingly as good. It has the effect of doubling the pulsation frequeny and halving its amplitude, which can in very many cases make the pulsation minor enough to live with. Then proceed with the torque tuning for even more benefit. The second reason brake discs pulsate is from uneven wear. This is not the same as warpage, which is, if you will, waviness. Disc thickness variation continues to be a major cause of brake pulsation even today. The corrective method here starts with recording the thicknesses of your disc at eight equidistant points around the disc. A difference from thickest to thinnest of a certain amount qualifies that disc for the rubbish bin, but you an still try the rephasing and/or torque tuning and see if they help, first. Can't hurt. A third and really very common cause of brake pulsation is patches of hardness change in the metal, at spots around the disc. Today's ultra-thin discs exhibit this problem more than do older brake discs. The corrective measure, since there is no warpage or thickness variation to resurface out, is really limited to replacement, after seeing how much rephasing and torque tuning will accomplish.
Disc Brake Pads
The demand for ever higher performing disc brakes has pretty much mandated the higher friction sintered or ceramic brake pad today. The problem is, in many cases this is at the expense of faster wearing brake discs, which on the early examples an be heavily gouged in not all that many miles of use. In fact the less aggressive organic pad is now quite hard to fiind. Manufacturers take pad type fairly seriously, usually allocating one type for example for their non-ABS bikes and another different pad type for the same model with ABS. Personally, on vintage bikes having the softer high stainless steel content brake discs, I prefer the much softer organic pads, which more slowly wear their discs.
The Effects of Storage
Most brake systems degrade somewhat during storage, a consequence that generates some little controversy on user forums. I have a brake fluid article on my site, take a look. Glycol based fluid can generally be counted on to trash a brake system that is stored for three years or more. The problem will evidence itself mostly at the master cylinder, whose tiny bypass hole will clog and from then on the brakes will self-apply. Of course you have all the other areas of the system to clean and rebuild, so this one item will not pose the major hurdle in your restoration, but it will keep the machine from rolling smoothly across your shop floor. My recommendation today is to use silicone based fluids in machines that see less than at least intermittent use, along with a good fuel stabilizer in the fuel tank and carburetors. Saves a heap of trouble.