Caliper Pistons not pushing in

[NickGreenhough]

1999 ABS / TCS

I wonder if anyone has come across this problem before.

The front brakes are sticking after I have replaced the seaks and the master reservoir seals as well. When I operate the brake the pistons freely move out. However, when I try to push one in, the other pops out. If I jam one of them and push on the other with the g-clamp, there is terrific resistance and fluid does not push back into the reservoir. The only way to get them back in is by opening the bleed valve. As far as I can tell the litte return orfice in the reservoir is clear.

The middle piston connected to the rear brake is fine.

I am hoping I don't have to strip everything down again and that it's a simple fix that I am just not seeing.

 

[dwalby]

I'm no expert on the topic, but from your description my first thought was you may have the master cylinder overfilled with brake fluid. After you move one piston a certain distance, there's no more airspace in the m/c reservoir to allow further compression.

 

[jfheath]

The 2 outer pistons are moving freely. So the problem has to be in the master cylinder or in the lines.

Have you tried the two outer pistons on the other side ? Careful - they are connected together, so you may end up pushing out the left hand pistons when you push in the pistons in the right hand caliper. (Which would prove that the line betwen the two calipers is clear).

The other considerations are at the master cyclinder end.

Check the level of the fluid in the reservoir (as decscribed above).
The compensation port - you have mentioned - but if that is blocked, you may have an issue.
Remove the brake lever - see if it happens then - it will remove the possibility that the lever is preventing the master cylinder piston from clearing the compensation port.

 

[John OoSTerhuis]

I suspect you installed the MC piston seal incorrectly (backwards). I could be wrong.

John

 

[jfheath]

I suspect you installed the MC piston seal incorrectly (backwards). I could be wrong.

I missed that he had had that apart. This is the most likely explanation. You have to ease the wide part of the seal into the bore first.

 

[NickGreenhough]

Thanks for all your replies and advice.
I losened the banjo bolt at the master cylinder end and they pistons could be pushed back. I had a spare master cylinder in the shed that was in pretty good nick so I put that on, bled the system and all is fine.
I suspect that I have indeed reassembled the seals wrongly in the master cylinder. I can play with that at my leisure.