caliper pistons withdraw back
Thread Starter
8th Gear
Joined: Jan 2019
Posts: 8
Likes: 0
OK, no knock-back when you are driving.
I'd guess it's one of two issues.
#1 - The master cylinder has lip seals inside. When the pedal is released the lips are supposed to fold in enough to let fluid go past them so they don't pull a vacuum on the lines which then pulls the caliper pistons back. Maybe your master has a lip seal that are too heavy or a spring and washer that is against the outer lip enough that it can pull a vacuum.
#2 - Something is different with your o-ring piston setups compared to most, and the o-rings are rolling into a loaded position so that they work like a spring and push the pistons back as they unroll once the fluid pressure is released. You saying you can hold the pistons when the master is released and they stay out tends to point to this not being the issue.
There must be clearance between the pushrod and the master piston. It shouldn't be too hard to determine how much you have by carefully moving the pedal and feeling when contact is made.
I'd guess it's one of two issues.
#1 - The master cylinder has lip seals inside. When the pedal is released the lips are supposed to fold in enough to let fluid go past them so they don't pull a vacuum on the lines which then pulls the caliper pistons back. Maybe your master has a lip seal that are too heavy or a spring and washer that is against the outer lip enough that it can pull a vacuum.
#2 - Something is different with your o-ring piston setups compared to most, and the o-rings are rolling into a loaded position so that they work like a spring and push the pistons back as they unroll once the fluid pressure is released. You saying you can hold the pistons when the master is released and they stay out tends to point to this not being the issue.
There must be clearance between the pushrod and the master piston. It shouldn't be too hard to determine how much you have by carefully moving the pedal and feeling when contact is made.
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
Look a METHOD A...line 4.....OR...look at METHOD B line 4.
https://www.performanceonline.com/bl...ench-bleeding/
Just so we are clear... because I have paperwork that comes with master cylinders that state the same.
SO...if your MC states something different... than do what they say due to THEY made it. Or play it safe and not press your luck.
Because I wonder if pressing the piston in too far when bench bleeding it may not cause the rear seal to leak fluid out the back...but the inner seals get damaged so you basically LOOSE your bias from the front reservoir to the rear reservoir. I have no clue if any of this is correct due to I do not push the piston in more than the paperwork states on the MC when and if I bench bleed it. Many items I just use my pressure bleeder and not need to bench bleed it.
DUB
https://www.performanceonline.com/bl...ench-bleeding/
Just so we are clear... because I have paperwork that comes with master cylinders that state the same.
SO...if your MC states something different... than do what they say due to THEY made it. Or play it safe and not press your luck.
Because I wonder if pressing the piston in too far when bench bleeding it may not cause the rear seal to leak fluid out the back...but the inner seals get damaged so you basically LOOSE your bias from the front reservoir to the rear reservoir. I have no clue if any of this is correct due to I do not push the piston in more than the paperwork states on the MC when and if I bench bleed it. Many items I just use my pressure bleeder and not need to bench bleed it.
DUB
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Interesting Dub, on the shorter 3/4 - 1" stroke. The MC I got from Corvette Central said 1 + 3/8. Of course I went about 2" before I realized I broke it.
I wonder what determines the different length strokes?
I wonder what determines the different length strokes?
Last edited by HeadsU.P.; Jan 6, 2019 at 06:05 PM.
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
I have not sat down with different designed master cylinders and see if the holes in the reservoirs are in different areas which I feel that is where the problem can occur if the piston is pushed too far in.
DUB
