Hi guys, new member here though not new Commando owner. Bike is put up now for winter but I still hope to be active at the forum through the snowy, deep, dark, cold, pot holed salt and sand covered road season. Of course I come with a question. My Mk3 has a stock frt master cyl, caliper, and rotor but with braided ss brake line. The system works well, nice hard lever if not a little wooden and good braking overall. This past season however the lever would slowly come back towards the bar under light braking or merely holding the bike at a stop. Heavy pressure, no problem, pull the lever as hard as you like and it will clamp and hold. I have rebuilt the master with no change. The bore shows no sign of damage. There are no external leaks at all, but that's exactly what it feels like is happening,.....but only under light application.
HI, with brake issue
- Thread starter Biscuit
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Welcome to the forum.
That is often a symptom of a worn or damaged piston cup seal. High pressure forces the 'cup' hard against the bore of the cylinder so the fluid cannot get past, but at lower pressures the cup doesn't seal as tightly so fluid can escape past the cup if it isn't in good condition.
Biscuit said:
That is often a symptom of a worn or damaged piston cup seal. High pressure forces the 'cup' hard against the bore of the cylinder so the fluid cannot get past, but at lower pressures the cup doesn't seal as tightly so fluid can escape past the cup if it isn't in good condition.
concours
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Biscuit said:
Hi, welcome. When you rebuilt the master, how well did you scrutinize the bore? It must be smooth, no scratches/pits. Most all of the vintage bike masters I've seen have some defects. It may be the nudge you need to have it sleeved down to the smaller diameter. I have seen some original replacements available as well. I was all set to sleeve mine, but it works as designed, so I cheaped out and opted to accept it as is.
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Here's another scenario. It's ugly. It's not the M.C. end of actions but the caliper end. The 2 pistons are made of steel and very susceptable to rust which slows down return actions.Winter is here so do the M.C. end of things but strip down the caliper too and fit stainless pistons. Use special tool and heat.
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Just throwing out ideas. Like stop the light brake usages and try hard, just joking. How old is everything too. When was the last time you tore apart the calipers ? 30 or more years ago ? Consider upgrading the m.c. piston to smaller as rubber swells preventing movements and you always need better braking. But take my advice and strip it all down this winter ,roadsalty time.
Since it's a fairly easy job I'll probably wait ' til the correct weather starts showing up again. Don't have a warm place to work really. I'll probably start with rebuilding the mc again with for sure Lockheed parts. I've never had an issue with the caliper though, clamps great and releases instantly so I've never had either front or rear apart in 46000 mi. (owned this bike since new) In that time I've probably rebuilt the mc two or three time to stop seepage evident at the dust boot, but everything is dry as a bone now. As I recall the bore looked absolutly shiny and perfect. It does seem plausible though that under light pressure fluid is getting past the primary cup but not the secondary one so no external leakage whilst heavy pressure seals that primary cup so no self relaxing lever... as L.A.B. said.
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