10. Rear drum brake: examination and renovation - K75, K75T, K75 C
- Adjust the rear brake (when necessary) and check the remaining thickness of brake shoe friction material as described in
- If the brake is to be overhauled, first remove the rear wheel. See Section 3.
- Note: Before starting work, use a vacuum cleaner to remove all traces of loose dust, or at least wipe away all traces of dust
using a rag well soaked in high flash-point solvent. The brake shoe friction material may contain asbestos which is toxic and especially dangerous
when inhaled as loose dust particles, Refer to the warnings given in the
Safety First section of this manual.
- Unscrew the adjuster nut to release the brake rod. If required, the pedal pivot can be dismantled after the pivot bolt retaining
nut has been unscrewed from behind the footrest plate. The pivot bush and sleeve can be checked for wear and renewed if necessary.
- Withdraw the retaining circlip from the brake shoe pivot and withdraw the brake shoes as a single unit, folding them inwards to
form a ‘V‘ to release spring pressure.
- To ensure that the camshaft is correctly refitted, use a hammer and a punch to mark the shaft at the gap in the operating lever.
Remove the operating lever pinch bolt, noting how the wear indicator plate is fitted to the lever, then carefully prise the lever off the camshaft
splines. Tap out the camshaft to the left, noting the sealing washer at its right-hand end, the two O-rings, and the flat washer at its left-hand end.
- If the brake shoe friction material is damaged, fouled with grease or oil, or worn at any point to a thickness of 1.5 mm (0.06 in)
or less, the shoes must be renewed. If the friction material is still serviceable, clean the shoes thoroughly using a soft ‘wire brush‘
that is free from oil or grease and remove any areas of glazing using emery cloth. Check carefully that the shoe ends are smooth and unworn.
- Examine the return springs with great care and renew them if there is the slightest doubt about their condition or if they show
signs of stretching, fatigue, or wear. Later models will have a rubber damper fitted around the rearmost (camshaft side) return spring to prevent any
risk of the spring vibrating to the point of breaking, a problem which also produces severe brake squeal. If a machine is found without a rubber damper,
one should be fitted on reassembly; since two different types of damper are available, the brake shoes must be taken to an authorised BMW dealer
for the correct type to be identified. Note that BMW also recommend that both return springs must be renewed whenever an unmodified machine is found.
- Clean thoroughly and check for wear or damage to the brake drum interior and the final drive housing (noting the warning above
about dust particles), the brake shoe fixed pivot and the camshaft and operating lever components. If any component is found to be faulty it must be
renewed. Check particularly the camshaft and pivot bearing surfaces and the fit of the camshaft in its passage through the final drive housing.
The camshaft sealing O-rings should be renewed whenever they are disturbed. If any sign of oil leakage is found the detective seal must be found and
renewed. See Chapter 8 - Final drive.
- Examine the drum surface. Scrape off any deposits of dirt, brake dust or rust and wipe the surface with a rag soaked in high
flash-point solvent. If the necessary equipment is available, measure the drum's inside diameter at several points to check for ovality; none should
be discernible. If the drum is heavily scored it can be reclaimed only by skimming on a lathe, a task which can be undertaken only by an expert who
has the necessary equipment, If the drum is worn at any point to the specified service limit or beyond, or if skimming will enlarge it to a similar
degree, the rear wheel must be renewed.
- On reassembly, fit the new camshaft sealing O-rings to their grooves and apply a coat of the specified grease to the bearing
surfaces of the camshaft and its passage and to the fixed pivot. Refit the camshaft with its flat washer on its left-hand end; ensure that it is well
greased, that the O-rings are not disturbed and that the sealing washer is pressed into place on its right-hand end. Rotate the camshaft so that the
alignment mark made on removal is uppermost, then tap the operating lever onto its splines so that the mark aligns with the lever split. If the shoes
are to be renewed, do not refit the pinch bolt yet.
- Hook the return springs on to the shoes from left to right ie so that the springs are on the left-hand (wheel) side of the
shoes. Apply a thin smear of grease to the shoe ends, fold them together and refit them to the fixed pivot and camshaft. Check that the shoes are
securely and correctly positioned then wipe off all surplus grease and refit the retaining circlip to the fixed pivot. Fit the rubber damper to the
rearmost (camshaft side) return spring so that the damper flat rear face touches the brake shoes and cam, and its chamfered side face is towards
the wheel. Refit the rear wheel.
- If the original shoes have been refitted, install the wear indicator plate and operating lever exactly as they were removed,
adjust the rear brake and check that the brake operates correctly and that the rear wheel is free to rotate smoothly and easily.
- If new shoes have been fitted, first check that the brake pedal height setting is correct
(see Maintenance - Major Service - 14-3. Check the drum rear brake) and connect
the brake rod to the operating lever so that the angle between them is more than 90° when the brake is correctly adjusted and fully applied.
See Maintenance - Major Service - 14-3. Check the drum rear brake
for details of this procedure, which may involve the repositioning of the operating lever on the camshaft splines.
- When the operating mechanism is correctly set and adjusted, position the wear indicator plate pointer to align with the upper
(Maximum) wear limit line on the final drive housing when the brake is fully applied. Hold the indicator plate steady while tightening securely
the operating lever pinch bolt, do not disturb the plate after this without first marking its exact position on the operating lever. Check the
rear brake adjustment and that the brake works correctly, also that the rear wheel is free to rotate smoothly and easily.