Bleeding front brake...spongy no matter what!

chris.p · 07-07-2020, 08:39 PM

No problem Bitza. I had meant to follow up with a better explanation and some pics, as I had the same problem when I fitted the 4 pad callipers to my 2000 748R, but a few things cropped up & it slipped my mind sorry. I am pleased you got it sorted, I had the same problems as you did fitting the radial master cylinder, but it does go on in the end.
Any way, pleased you have resolved the issue.

Mr Gazza · 07-07-2020, 09:24 PM

After the event and now you have solved the problem, the penny has dropped as to what the problem was.

I think that you were most probably getting all the air out and bleeding successfully, but the increased lever movement and light action was due to the larger caliper pistons demanding much more fluid movement. Much more than the 16mm master piston could deliver in the stroke available. So you never got a hard feeling brake lever.

Now that you have converted to probably something like 19mm master? It can move sufficient fluid with less lever movement.

Sorry to be so slow off the mark. I clearly did not register that your caliper pistons were larger than before.

Glad you are sorted now..

07-07-2020, 09:24 PM	#2
Mr Gazza Lord of the Rings Join Date: May 2013 Location: Norwich Bike: M900sie Posts: 5,848	After the event and now you have solved the problem, the penny has dropped as to what the problem was. I think that you were most probably getting all the air out and bleeding successfully, but the increased lever movement and light action was due to the larger caliper pistons demanding much more fluid movement. Much more than the 16mm master piston could deliver in the stroke available. So you never got a hard feeling brake lever. Now that you have converted to probably something like 19mm master? It can move sufficient fluid with less lever movement. Sorry to be so slow off the mark. I clearly did not register that your caliper pistons were larger than before. Glad you are sorted now.. __________________

07-07-2020, 08:39 PM	#1
chris.p Gold Member Join Date: Mar 2004 Location: lincoln Bike: No Bike Yet! Posts: 876	No problem Bitza. I had meant to follow up with a better explanation and some pics, as I had the same problem when I fitted the 4 pad callipers to my 2000 748R, but a few things cropped up & it slipped my mind sorry. I am pleased you got it sorted, I had the same problems as you did fitting the radial master cylinder, but it does go on in the end. Any way, pleased you have resolved the issue.