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Front brake!

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PatM:
Hi All!
I went for the MOT today after my rebuild and failed on front brake binding and the throttle cable snagging on full-lock. One is an easy fix- but the brake is a b'strd.
I fitted new pads and yep, it does bind- but as they are running in , I left it. the non moving pad has only just got a kiss on it and the moving- I cant tell (as Ive only done about 10 illegal miles, fettling- without an MOT..)
I have had problems with 400/4 brakes in the past- trying to get a good brake on a 400 is a black-art Ive never mastered- but the only way I can reduce the bind on this is to either loosen the capiper (err, a non, no) or fit the moving old pad.
The brake is still crap, but it might just do...as the rear locks a treat!

any comments/help or jibes, feel free!

Rgds

PatM

honda-san:
When you fitted the new pads, did you pump out the piston and clean out the groove in which the seal is fitted? Corrosion tends to "grow" behind the caliper seal and squeeze it onto the piston, preventing it from returning properly. Often not helped if the piston is just pushed back into the caliper after fitting the pads as any dirt on the piston is forced towards the seal.
Other problem area can be if the tiny bleed hole inside the master cyclinder is blocked, not allowing brake fluid an easy retrun to the master cyclinder after squeezing the lever - use a pin to clear it.

These things are pretty old now - only solution with them is to strip, thouroughly clean out the caliper (especially the seal groove) and re-build with new fluid, etc. I use rubber brake grease behind the piston seal, and DOT 5 silicon brake fluid (doesn't burn the paintwork if it spills / leaks). Old pads have often glazed, and have been known to detach from the backing plate, so would certainly fit new ones if in doubt (which of course you have). Corroded pistons, perished brake pipes should be replaced too.

Obviously new pistons, seals, pipes ets are desirable, but even with the original parts (subject to condition of course) these brakes can be made to be pretty good even if they OF COURSE are never going to compete with the latest generation 6 pot twin disc set ups!

Have fun! - Chris R.
 

K2-K6:
Good response from Chris, covers just about everything you should be doing.

To add a bit to this about the seal and how it works: I believe the seals of this type deform in cross section as the piston drags out past them it is dragged further out at the piston contact point, when you release the brake pressure the seal elasticity? pulls the seal back to it's original shape and has the effect of taking the piston with it back into the caliper so giving a small pad to disc clearance and hence no binding.

It's vital to clean the seal area as chris says to restore the tolerances that enable this to happen. Only a small amount of muck/corrosion on the pad side of the caliper seal effectively blocks the space for the seal to operate as designed so it's critical. also I have always been able to return bike caliper pistons into the bore with two thumb pressure, it's a good measure of the piston not binding.

Bryanj:
Main problem used to be the pivot pin siezing

honda-san:
And of course, as Bryan suggests, make sure the caliper arm pivots freely - loosen the caliper bolts, the arm should pivot freely with the spring capable of moving it. Fitted a grease nipple to my winter hack 400/4 caliper arm so the pivot can be greased - no more seizures.

Chris

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