I suppose with an older '97 K1200 RS ABS II it was inevitable that one day I would be into the brake master cylinder, but I did not expect to find a problem this way. For a couple of years I have had this feeling that the front brake pull on the first pull is further back than it is on a second pull immediately after. I kinda got used to using two pulls to brake, instead of one and never had a problem stopping the bike.
I have just replaced the oem lever micro switches with brake line pressure switches and added a 'brake on' led in a spare location in the instrument warning lamp array. The brake switches I used come on at a higher pressure than I would like, but here is the scenario:
I am slowing gently using only the front lever and there is sufficient fluid pressure to put on my warning lamp and rear brake light. Over a few hundred yards I expected it to stay on, but it went out! I repeated the same slow down, this time pulling on more lever pressure and I could swear the lever was moving slowly towards the throttle grip - difficult to say when riding.
I have just been through the entire braking system and I know calipers/pads and unions are all OK. I get back home and check everywhere for a fluid leak and there is none. The master cylinder reservoirs are as full as when I topped them off the previous day. So, brain in gear the master cylinder must be losing pressure.
I put the bike on the stand and give a gentle pull on the front brake lever just putting on the brake light (nice to have the led now in the cockpit). I held the lever for 30-40 seconds in the same position, the light stayed on and the lever did not move. Then I put a lot more hand pressure on the lever pulling it back to about 30mm from the throttle grip. Sh*t, over 30 seconds the lever was slowly moving back till it hit the throttle grip and my brake warning led went off!
I got a fair deal on a used MC off a newer bike today and that gave me chance to look further before buying a MC repair kit to repair my original MC.
I tore down the suspect MC, pulling out the piston with its seals and return spring (see photos). The MC bore looked fairly clean apart from a couple of spots where the alochrome has taken some wear. The secondary seal at the back looked in poor condition, but that does not do all the pressure work. Even so, it could be responsible for pulling in air at the back and causing my lever to always seem soft, however much I bled the system.
The front primary seal does all the pressure work. I had a look at the seal edges under a 30X microscope and there were uneven 'flats' around the edge. That would explain the slow leak back of fluid back to the MC reservoir. I have been riding this bike nearly 2 years with annual DOT tests and never had a problem stopping. But If I had to do a long stop from high speed, I would have had the lever hit the throttle grip as the system leaked out pressure.
I can only advize you to check your front and rear master cylinders can hold pressure for at least 40 seconds or more. If either the pedal or lever changes position, or the 'feel tension' appears to change, then check your master cylinder seals. Do not assume you just need to bleed your brakes!
I don't have servo brakes, but somehow you need to hold in the brake lever or rear brake pedal and feel if the pressure is sagging over 40 seconds or longer. Perhaps you can disconnect the servo to make the check more sensitive.
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