Disc. Scored & worn below recommended limits (sorry no photo), a new item was sourced along with OEM Yamaha hex bolts. That was easy, however getting the old one off wasn't. The Haynes manual states that the 'bolts can prove difficult to remove' no kidding! You will need the use of the following:
Heat the bolts up a lot (this didn't damage the paint on the wheel amazingly) & go hell for leather with the ratchet driver fitted with a good quality snug fitting hex bit. I got lucky with 5 but the 6th one sheared & had to be very carefully drilled out & re tapped. Use plenty of Locktite on reassembly so the next person to replace this has the same problem. Tighten the bolts down gradually & evenly to avoid warping the disc.
Calliper. I don't think this had been serviced for a very long time. The pistons were almost seized, but popped out eventually. The dust seals had long since dissolved, the pressure seals were not too bad , but the seal seats were encrusted in a hard calcium like substance (borate so I am told). The bleed screw wouldn't budge & sheared off flush with the calliper. Having read other horror stories about unsuccessfully removing these I had an idea... One of my other hobbies is renovating old watches & sometimes steel stems shear off inside the crown:
I have removed these by soaking them for a few days in a strong solution of Aluminium Potassium Sulphate better known as Alum:
This will dissolve steel but not aluminium, brass, copper etc. It also works faster when heated. So a strong solution was mixed up in a Pyrex bowl with the calliper immersed into the solution this was then placed inside a saucepan with water in it, put on the stove & set to a slow simmer:
Note the bleed screw (red circle) fizzing as it dissolves. After about 50 hours of simmering the bleed screw finally dissolved - the same area circled on the new screw measuring 12x6mm, with no adverse affect on the aluminium calliper apart from a slight discolouration:
No drilling, damaged threads or seat. Unfortunately I could not find any chemical capable of removing the borate deposits without damaging the calliper, so I had to carefully scrape these out & clean over with a Dremel fitted with a circular wire brush. Finally the calliper was given a good coat of PJ1 satin black & when dry baked in the oven at 250 centigrade for 1 hour:
This should make it very resistant to brake fluid. New seals, pistons & a stainless steel bleed screw were obtained & fitted, along with a new Hel hose:
Master Cylinder. Paint peeling, plastic sight glass cloudy & leaking (& no photo again) but otherwise functioning. The sight glass had to be removed so the unit could be painted as per the calliper. This was pried out easily enough with a small sharpened screwdriver, but as we know originals are not available. After-market kits can be obtained from the other side of the world, but being a bit impatient, an 18mm diameter 3mm thick glass watch crystal was ordered from: https://www.ebay.co.uk/itm/Watch-Glass- ... 2749.l2649
The glass was initially fitted in place with Araldite & when dry black Milliput 2 part epoxy putty was applied & smoothed down with a wet finger. Here's the result:
The brake was duly filled with fluid (a painfully slow process) bled, checked for leaks & now works like new. The moral of the story? - DO bleed & service the brakes & DO change the fluid periodicaly.