Grimeca Rear Brake Master - 1980 500S

Maestro, SEI-V

Grimeca Rear Brake Master - 1980 500S

Postby schrammel » 16 Nov 2016 20:07

Hello,
I am in the process of rebuilding a rear Grimeca master.
I replaced the o-ring (1st groove) and the U-cup (3rd groove, closest to spring) with the open/pressure side facing the spring.
My problem: after reassembling the master piston moves more or less freely and the spring pushes back the piston after manually compressing it.
After letting it sit for about 1h without moving it the piston appears to be stuck in the cylinder. I can hardly compress it anymore and need quite a bit of force to do so.
After being compressed the piston is stuck and the spring is not pushing it back.

Any idea what's wrong and how to fix it?
Thank you!
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Morini 1980 rear master Grimeca.jpg
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Re: Grimeca Rear Brake Master - 1980 500S

Postby MickeyMoto » 16 Nov 2016 20:55

Have you filled it with fluid or is this before filling?
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Re: Grimeca Rear Brake Master - 1980 500S

Postby schrammel » 16 Nov 2016 22:08

This is in a dry state, master not installed and without brake fluid, just applied a thin layer of brake grease on the o-ring and u-cup.
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Re: Grimeca Rear Brake Master - 1980 500S

Postby Manda » 16 Jun 2018 22:09

I know this is now old, but did you resolve the sticking problem when it had stood for a while.
I have just rebuilt a rear m/c in the same manner as you and am also getting binding, was wondering of the u-cup should be in the other groove.
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Re: Grimeca Rear Brake Master - 1980 500S

Postby Ming » 17 Jun 2018 06:25

Sounds exactly like the problem I just had on a K2 - everything suddenly went stiff (ooer) and when I disassembled it one of the seals had a small tear. After replacing the seals everything OK again, but I ALWAYS lubricate the new ones and the bore of the cylinder with clean brake fluid on reassembly.
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