Sunday, March 18, 2012

VFR Caliper Rebuild

After having a go on Helen’s VFR last year I realised how bad the brakes on my VFR had actually become. So I decided that the constant glazing of the front discs was most properly due to the pistons binding. The discs are quite pitted as well but I really don’t want to be buying some new discs when I am likely to replace the VFR next year. So I decided the best course of action was to strip down the calipers and replace all the seals and then to fit a new set of brake pads to the fronts.

I removed all the brake calipers and then slightly ejected all of the pistons. I then sucked out all of the brake fluid with a brake vacuum pump. I disconnected the banjo bolts and then ejected the pistons from the caliper bores with compressed air. All of the pistons were slightly pitted but looked in good condition otherwise, bar one off the rear caliper that was heavily scored. The caliper bores and seals looked in good condition, so I cleaned the bores and fitted the new sets of seals anyway. I also decided to replace the heavily scored rear caliper piston. I then re-assembled the calipers and refitted the pads and re-installed all the brake pipes and banjo bolts. I then refilled the brake fluid and sucked it through with the brake vacuum pump. I would advise that you purchase one if you intend to do this job as you will struggle to get the fluid through without one. Once the fluid was through it was time to bleed the brakes, I started with the fronts as you can do this yourself. Do the left caliper first (top bleed nipple) then the right hand caliper, you can now seal the front brake reservoir.

Now for the pain in the arse rear circuit, you will need a second person for this. You need to remove the front left caliper and put something in between the pads to stop the pistons being ejected. You then need to bleed at the proportional valve by the battery, while someone activates the secondary master cylinder pushrod at the left front caliper. Once that is clear it’s time to bleed at the rear caliper middle bleed nipple, while again activating the secondary master cylinder pushrod. The Haynes manual advises you to remove the caliper and re-mount it the other way round on top of the disc, to make the bleed nipples accessible. I found even doing this I was unable to get all the air out. The solution is to remove the caliper and place something between the pads and then rest the calliper on top of the disc with the bleed nipple pointing directly upwards, only then did I manage to get all the air out. Once that is done you need to bleed the rear pedal circuit. First is the servo proportional valve (right side under seat by fuel tank), then the rear outside bleed nipple and then the front left lower bleed nipple. If after this the rear pedal is still a bit spongy there is more than likely still air in the secondary master cylinder circuit. Unfortunately due to the MOT having lapsed on the VFR I am unable to test the brakes properly till I have taken it in, I will update this post after that point.