Brake strip

The 1991 Trophies are fitted with disc brakes operated by Nissin single-sided twin-piston calipers. When the pistons are forced against the brake pads by hydraulic pressure, the calliper bodies slide outward on two pins that are fixed to a mounting plate.

It is pretty common for this sliding pin arrangement to suffer from seizing up. In part, this is because of corrosion that builds up around the rubber boots that are supposed to keep them clean. The boots themselves are typically very swollen due to contamination with mineral grease and so press on the pins. Diligent owners may use copper grease on the rear of the brake pads and pad retaining pins. Unfortunately, copper grease is mineral based and so is bad news for the rubber components. The rubber boots need either grease made from vegetable oil (normally caster oil based 'red rubber grease') or silicon grease.

The callipers front and rear are the same type, sharing 27mm pistons. In fact the calliper bodies for the rear brake and one of the front brakes are identical - only the mounting plates are different. Period road tests described them as adequate rather than good. Later Trophies were fitted with more powerful Nissin double sided four-piston callipers, as used on the original Daytonas and then on the later T5 Sprint models.

Nissin twin-piston front brake callipers

I found the all three callipers on my bike to be in fair cosmetic condition but with split dust seals. The pistons were very very tight in the calliper bodies, suggesting that corrosion had built up behind the hydraulic seals. So a split, clean and seal replacement was needed.

Starting with the rear brake, the master cylinder appeared to be in reasonable condition but with a rusty pushrod, banjo and brake switch coupling.

Nissin rear brake calliper and
master cylinder

To strip the rear brake master cylinder, it is first necessary to remove a rubber boot that is supposed to keep dirt and water out of the joint between its push rod and body. The seal is held tight against the body by a spring. When I managed to pull the boot out, I was greeted by a horrific rusty slurry. Once again, we are seeing the horrible result of using a pressure washer on a motorcycle. The boot itself was not holed or loose. In the picture below, it appears that the spring had ripped out of the boot. However, I cause this damage when I pulled it out. The only way the water could have got in - and then be retained - was if it was forced in there by a pressure washer. Worse still, the piston inside the master cylinder is retained by an internal circlip. One of the circlips eyes had completely rusted away, meaning that I could not remove it with my circlip pliers. My only option was thus to find a serviceable replacement - there will be a future posting on that.

Stripping the brake callipers themselves presented no further surprises. I had pumped the pistons almost all the way out of the calliper bodies before disconnecting the brake pipes. Even so, it was very hard work freeing them off the final two or three millimeters. I used self-gripping pliers cushioned with scrap rubber strips. This is something that must be done with extreme care - use the absolute minimum tension on the pliers to maintain a grip through the rubber strips and a gentle rocking and pulling motion. Patience is the key thing and a risk that the pistons will be damaged in the process. I managed to do it without causing any further damage to the pistons.

As expected, the dust seals were very chewed up and there was a layer of white alloy corrosion behind all the seals. The seals came out easily enough using a very small and blunt flat-bladed screwdriver. Patience and a twisting motion will be your friends. The sliding pin dust boots were variously ripped and swollen.

Rusty sliding pin protruding into ripped
rubber boot

One front calliper stripped down to component parts. Listed in
anticlockwise order from the calliper body, they are:
hydraulic and dust seals, bleed nipple, mounting plate, slider pins,
rubber boots for slider pins, and 27mm brake pistons.

Three stripped and cleaned callipers hanging in the sun to dry