Roadholder 364 - April 2018

29 The washers were placed to allow the inner piston to move out. I used this plate and washer arrangement with the calliper still attached to the fork leg and pumped the front brake lever, having placed a container below the calliper to catch the brake fluid. The inner piston pushed out. After removing the end plug I was able to push the outer piston in past the seal, remove the outer seal with a needle pick, push the outer piston out using a screwdriver in the calliper gap (normally occupied by the disk and pads) and then used circlip pliers expanded inside the inner piston to remove it. The calliper internals were cleaned using brake cleaner and showed little wear. The outer piston was not in bad condition, although starting to rust. The inner was corroded on the surface which probably explains its reluctance to retract correctly. Photo of the inner piston shows the corrosion: Both pistons were replaced with new stainless steel ones from RGM. For the inner piston, I chose to use the modified one RGM offer with a threaded centre boss to aid removal (Part No. 061896M) shown in the following photo: The piston boss has a thread of 5/16" UNF and will help a future owner remove the inner piston more easily if it becomes stuck or heavily corroded. The inner piston goes in closed end first and the outer piston open end first. New piston seals were pre-soaked in brake fluid for around one hour to help ease piston fitting. The seals are square in profile and can be inserted either way round. The seal groove is machined with a slant in the bottom of the groove and this gives the seal the correct taper when installed. After fitting the inner seal to the calliper I used a 3 5 / 16 " UNF bolt to help locate the inner piston at the correct depth. It is all too easy when pressing this piston past the seal to go too far. The depth needed is just enough to allow the pad to be seated. Too far might block the channels for the hydraulic fluid. The only issue I had was getting the inner piston to pass the seal. I have suggested to RGM that a bit more chamfering would help. The outer piston located easily after fitting the new outer seal to the calliper. A new 'O' ring was added to the end cap before fitting. The calliper was than refitted and connected to the bundy pipe, pads inserted and wheel fitted. New DOT4 fluid was added and the brake bled, finishing with the usual trick of cable-tying the front brake lever against the handlebar overnight to complete bleeding. The brake and calliper are now working fine.