Topic: To grease the caliper piston & seal... or to not grease

[Kickstart]

I'm rebuilding a front brake caliper on my 75' CB750F, and I wanted to know what is the proper way to install the new piston:

Here our my options:

• Just push the piston in dry (like I did on the last caliper I put back together... was that a bad move?)
• Wet the seal with brake fluid (I believe this is mentioned in the brake FAQ)
• Lightly coat the piston, and maybe the seal as well, with silicone grease (I believe this was mentioned in another thread)

Knowing that the seal "grips" the piston, and retracts the piston back after the pressure is released... I would think that I wouldn't want much (if any) lubrication between the piston and seal.  Although I can see how putting the piston in dry may damage the seal.

Thoughts?

Thanks,
Chris

[TwoTired]

You do NOT want silicone on that seal.  It will contaminate the brake fluid.  Not good.
The proper fluid is brake parts assembly lube (not commonly found anymore, you have to look for it).  It is totally compatible with DOT3,4, 5.1.  So, it is ok if it get on either side of the seal.
Brake fluid is often used for assembly, which is better than dry.  However, there will be some on the pad side of the seal. Since brake fluid absorbs water, you can expect faster corrosion of caliper and piston.  The assembly lube doesn't absorb water.
You'll note that water can easily get behind the brake pad and help corrode all the metal behind it.  That is what the thin coating of silicone on all the exposed, unpainted metal bits is for; it sheds water away from the metal and it can't wash off, keeping the caliper healthy and operational.

Cheers,

[Kickstart]

Thanks TwoTired,

I also see that you have previously answered the lube question in a previous thread (as well as described which lube to use where)... and you also mention that the assembly lube does not interfere with ability of the O-ring to grip, and retract back, the piston:
http://forums.sohc4.net/index.php?topic=56544.25

I was able to find both the Silicone grease and assembly lube on Amazon (including the link for the benefit of others):

Dow Silicone High Vacuum grease:
http://www.amazon.com/gp/product/B0017XZU18

Brake Assembly lube:
http://www.amazon.com/gp/product/B001ANJ2GE

When applying the silicone grease, should I install the piston first (pushing it in all the way) and then coat anything still exposed with the silicone grease (including the exposed, dry side, of the piston)?

Also, should I coat the entire O-ring with assembly lube prior to putting it into the grove, or should I just put a light coat of assembly lube on the inside of the O-ring after I install it in the grove.

Thanks for you help,
Chris

[TwoTired]

I coat the whole caliper seal with a thin coat of brake parts assembly lube.
The goal is to coat all the bare metal on the puck side of the caliper seal with silicone.  Metal piston, caliper bore, and brake pad backing.
You can dry fit all the parts of the caliper without the seal to get a clue how deep the piston fits into the caliper while in operation.  Then coat the metal that you know will be exposed so it can't corrode.  You don't need gobs of silicone.  More is not better.  Just a thin coating will do.  It isn't going anywhere, unless it is physically and forcibly displaced.

Cheers,