Rebuilding the brakes

In my previous post, I wrote about the horrible condition of the rear brake master cylinder. I bought a second hand replacement. When it arrived, I found that it was in great condition (hooray!) but that the design was slightly different (strange). The component sizes were all the same - the difference was in the push rod. 

Old (left) and replacement (right)
master cylinders. Note the special
nut fitted to the push rod on the
replacement master cylinder

Comparing the condition
of the replacement (right)
and original (left) master
cylinders after removing their
rubber dust boots.

The push rod of the replacement master cylinder has an additional short threaded section just before it enters the cylinder body, onto which a special nut was fitted. The nut has a recess machined around its base to locate the lower aperture of the rubber boot. I'm not sure why this design change was made but my best guess is to make adjusting the length of the pushrod easier. 

The lower end of the push rod winds into a threaded hole in bracket that attaches to the rear brake pedal. The pedal's height is adjusted by winding the push rod further in (to lower the brake pedal) or further out (to raise the brake pedal). The push rod is then kept in place with a lock nut. I found that the push rod on my original master cylinder was seized into the bracket and therefore that I could not adjust the height of the rear brake pedal when I released its lock nut. I could not get a good grip on the push rod to try to rotate it into the pedal's bracket. So I think the nut design is to help with the business of winding the push rod in or out. 

Replacement master cylinder stripped down to its
component parts. From top right anticlockwise, these are:
threaded brake pedal bracket, locknut, rubber boot,
special nut, special push rod retaining washer,
push rod, master cylinder body,
master cylinder return spring and seal,  hydraulic piston,
and finally a circlip to keep the  retaining washer in place.

Having completely dismantled three callipers (two front, one rear) and the rear master cylinder, I next cleaned up and repainted the various bits of hardware involved. Clean up was effecting with my trusty electric drill, wire brush and patience. The picture on the right shows the result - very pleasing. 

Brake calliper mounting brackets
after painting

I had a tin of BRG paint I'd never used. I can't remember where I  bought it from or why, probably an unmissable bargain at the time. The original calliper mounting brackets were a kind of olive green - I know because the previous owner had bought a new one for the rear calliper. The front brackets were coated in a mixture of brake dust and rust. As usual, all steel components are primed with a zinc-based paint. The BRG turned out to be pretty but hard to use. It needed lots of stirring, took ages to dry and I am doubtful about its durability. Oh well. 

The main sticking point with the design of these twin piston callipers is the sliding pin arrangement, as discussed in my previous posting. The are prone to sticking. I decided on a multi-prong attack on these items to try to beat the problem. First, I polished all the pins to a high finish using my drill with fine grit and then polishing mop from my Cetem kit. I've had this kit for about 15 years and it has been fab for the odd bits of stuff I do. I've replaced a couple of the mops but still have the original bars of polish. Second, I bought genuine Nissin pin boots from Sprint. Per my previous posting, the boots on my callipers were predictably horrid - swollen and split. Thirdly, I treated myself to a big pot of Rock Oil Red Rubber Grease (oo er) from Wemoto - a tenner well spent I think - to lavish all over all things rubbery in general. More to the point, all over the sliding pin boots in particular.

Original calliper pistons after
buffing up with my polishing mops

 Turning to the calliper pistons, they looked, well, just plain manky. The pistons are chromed steel parts, normally with a coppery finish my pistons were more of a scratched grey colour where they avoided the russet tones of the dreaded eff-ee-two-oh-three. I couldn't tell whether they were serviceable or not after a regular clean so I buffed them up on my Cetem polishing mops. I was amazed at the transformation. They still showed some damage around their open ends, presumably where one or more previous owners had scarred them with pliers etc. in attempts to free off the brakes. Only two showed deeper problems in areas of the piston surface that would slide over the hydraulic seals. So I needed two new pistons. I don't know if Meatloaf's song would work as four out of six ain't bad, but the ratio's the same.

Two stainless steel 27mm pistons on the right,
damaged chromed steel originals on the left

Checking the dimensions with
vernier callipers - all fine.

I noticed a gent (vfrpaulvfr - needs a new username!) on Ebay selling stainless replacement pistons for the rear calliper of fuel injected Triumph Tigers. I did a bit of reading around and found the they are also 27mm in diameter. I didn't know about the depth of the pistons but took a chance and ordered them from him. On arrival, it turned out that the depth was the same too - 31mm. These are very nicely designed alternative parts, including a small step towards the closed (inner) edge to ease fitting. They are well made too. The bore is a bit rough but that's mainly a cosmetic issue (dirt will accumulate there) - the key thing is to do with the sliding surface and dimensionality. That's all great. So I'm very pleased.

I repainted the calliper bodies in two stages. Priming with Special Metals Primer takes care and will want lightly rubbing back afterwards because it doesn't flow terribly well. It does however provide good purchase on non-ferrous metals for a protective top coat. I followed it up with Hammerite Direct to Galvanised Metal in smooth black as a top coat. This is actually easier to work with than Smoothrite - it is recoatable much sooner than the mysterious and magical Smoothrite stuff. It is unlikely to be as tough but I think it can be more easily touched up.

So all parts refurbished or replacements organised, on to reassembly.

Repainted and lubricated, replacement master cylinder
now ready to go.

With the red rubber grease suitably applied, it was a simple matter to install the calliper pistons. The bores must be clear of rubber grease - it is for the dust seals. I used Dot 5 brake fluid in the bores to help the pistons slide home. 

The finished rear brake calliper is proudly displayed for your delectation below. I bought a new stainless steel pad spring - Triumph gen parts bag visible behind. The original was broken in two (!) for utterly mysterious reasons. Copper grease is applied to the rear of the spring where it clips to the calliper body and into the body where the larger of the sliding pins locates but clear of the rubber boots.