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front brakes sudden drop in pressure

5.1K views 5 replies 4 participants last post by  white wings  
#1 ·
so i jump on my bike today and as i was leaving my parking lot i noticed i could pull the brake lever further down than i used to, although its still more than enough to stop the bike. i checked my lines and calipers and i have no leaks, and it felt nice and firm the day before. anyone have any idea what might have happened? maybe a bad bleed from when i rebuilt my brake system and some bubbles came up? thanks for any help.
 
#3 ·
yes non abs. so ive since rebled the system twice since ive made this post. and about 3 days later the pressure will gradually drop and loosen up. when i bleed from the master cylinder a whole bunch of air comes up. the strange thing is i went for a hard ride in the twisties yesterday and through the continuous pumping of the brakes it firmed back up. could i still have had that much air still in the system coming up or is there a air leak in the system somehow? im absolutely positive theres no fluid leaks in the system.
 
#4 ·
If you dont do it properly, yes...there will be air trap inside. If you dont see any leaks from the system....there shouldnt be any air drawn in.

Bench bleed MC again...using jumper hose from fitting to the reservoir untill no more bubles coming out, sometimes need up to 20 strokes. And dont pump it fast. Slow steady motion and pause in between strokes are the best. Give a sec pause. And after no more bubbles....then calipers. Depends on what lines you are using....basically from closest to the furthest.

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#5 ·
I had a similar problem a couple of years ago, where the front brake would get spongy and when I would bleed it. I would get some air out and it would firm up for several days and then the problem would re-occurr. This was going on with my CBR and my RC51. I eventually traced it down to the brake fluid I was using. I bought a fresh supply, purged and re-bled the systems and the problem went away.

A trick I picked up years ago is when trying to get the last of the small bubbles out, was to bleed the system at the highest point, which is the master cylinder. Before they put bleed screws in the master cylinders, I would pump up the system, and strap down the lever to maintain the pressure. This compresses any air down to very small bubbles. I would then while holding a rag around the upper banjo fitting, crack the bolt to release the pressure an what little air that was still in the system.

I use the same technique on the newer systems, using the bleed screw on the master cylinder and get the same results.