Any reason not to use silicone grease to lubricate brake pistons

Was going to use Hunters silicone lube (as recommended by Hope) on Formula Cura brakes to help improve lazy piston/pad rubbing. Most data on the Hunter lube says it is DOT fluid compatible. Nothing about whether it is suitable on mineral oil systems (ie the Cura’s). In theory the lube/brake fluid should never meet so would have thought it is not a problem.

In theory the lube/brake fluid should never meet so would have thought it is not a problem.

That’s right, but the compatibility will be tested against the piston seals, which your grease will be in contact with. There are different materials for DOT fluid and mineral oil, probably Viton for DOT and Nitrile for mineral.

Don’t know the detailed answer to the question, you’d probably need to ask the lube manufacturer.

I’ve just rebuilt my Guide calipers and the instructions were very explicit in not using grease on the piston seals as it impedes proper operation.

I would use a smear of brake fluid (as per the Guides) – pump the pistons out, apply fluid with a cotton bud, push the pistons back, repeat until they move smoothly.

I would use a smear of brake fluid (as per the Guides) – pump the pistons out, apply fluid with a cotton bud, push the pistons back, repeat until they move smoothly.

This ^ is the best way to do this job :o)

Formula recommend PFPE grease ( Krytox).

I also use fluid.

I don’t think the pad is meant to slide too easily through the seal is it? It needs to bind a bit so that the seal flexes and then pushes the piston back in when you let go of the brakes…?

Brake fluid all the way. There is a really good GMBN Tech YouTube vid on brake setup hacks on this which is worth a look.

Don’t use grease, as molgrips posts above hydraulic brakes work by the seals deforming under pressure then returning to their original shape once you release the lever. The pistons are an extremely tight fit in the seals and only slide through microscopically as the pads wear. Sometimes they get stuck if you grab a huge handful of brake and they move through and jam on.

https://www.rideformula.com/wp-content/uploads/2017/01/Manuale-Cura-ENG.pdf

https://www.bike24.com/p2281514.html

Id argue that on a car (or motorbike) with big inflexible disks that the seals do work like that. On a mountain bike with thin disks that flex all over the place any resistance in the seals results in that side not moving. Probably easiest to make both sides move too much than it would be to get every seal moving just right.

I would never mix anything for silicone DOT systems with anything for hydraulic oil systems and vice versa.

However, if you are using typical DOT brakes then cleaning and lubing the pistons with that silicone grease works really well. The Hope tech video shows clearly how to do it.

Looks like Formula do recommend a ‘base PFPE grease 106205’ – probably Krytox. Not so easy to source though. Might start with a piston clean, followed by the application of mineral oil, and see how it goes.

I’ve been using durex lube (really) on pistons for years, on both dot and mineral. Seems to work perfectly

Id argue that on a car (or motorbike) with big inflexible disks that the seals do work like that. On a mountain bike with thin disks that flex all over the place any resistance in the seals results in that side not moving. Probably easiest to make both sides move too much than it would be to get every seal moving just right.

Not really. The pistons move when the seals flex, which is inherent in the seals. No grease required for that as nothing is sliding through anything. This is why on old brakes sometimes they don’t both have the same amount of throw, because the seals have stiffened up a bit differently.

On a car, the pads don’t really retract to clear the disc much. On an MTB they do, cos we don’t like rubbing. You can see the pads move back as you let go of the lever – it’s the seal doing this.

I’ve never greased them.

https://www.ebay.co.uk/itm/Formula-Piston-Kit-Pump-Piston-Kit-the-One-T1-R1-R1R-MY10-to-MY15-FD40116-20/401788946140?hash=item5d8c7cc6dc:g:zqsAAOSwjTlZfPaG

I mostly just lube them with brake fluid when changing pads – carefully pump the pistons out with the pads removed and dribble a couple of drops of the correct fluid onto the piston, then gently push it back in. I have some very old Hayes HFXs that have sticky pistons on my commuter bike. I pop the pistons out and use some Nissan branded brake grease whenever I change pads. Just seems to help the pistons reset themselves a bit more easily. They’re close to 20 years old, but still work fine once the pistons unstick themselves.

Hope also make mineral oil compatible calipers for Shimano road brakes. You could ask them what those are assembled with.

Don’t use grease

If argue against that as a blanket statement. Both hope and formula recommend a specific grease so there is no reason not to use it. Likewise if SRAM say don’t use it then don’t. The manufacturer after all knows their product best.

For 4 piston brakes it’s essential to lube the seals with something otherwise it’s very difficult to get them all moving evenly. I use brake fluid on my hopes but now I know there is a grease I’ll probably change to that as fluid doesn’t always seem to last long.

Seems like opinion is split on the grease or not to grease. Probably best if I contact Formula and get their advice from the horses mouth, and hopefully a source for the grease.

My last brakes were Shimano with loads of pad retraction. The Cura’s have a lot tighter clearance so need more fettling to keep the annoying pad rub at bay.

I’ve always used silicone spray. Makes it easy to get it into to the seals and cleans off easily with IPA.

Makes my Guide RSs run very smoothly

Not really. The pistons move when the seals flex, which is inherent in the seals. No grease required for that as nothing is sliding through anything. This is why on old brakes sometimes they don’t both have the same amount of throw, because the seals have stiffened up a bit differently.

True, but with a flexible disk and longer piston travel having one of 2 or 4 pistons sat further away from the disk is more of a problem than on a car where the piston barely moves.

Also, mountainbikers don’t have dust seals, cars generally do. So cleaning the seals on a bike is something that needs doing whereas a car they’ll last hundreds of thousands of miles.

From Formula tech dept:
– don’t grease pistons
– try try re-bleeding with disc IN caliper.

I had some sticky piston issues with Shimano brakes in January after total neglect for 6(rear SLX)-8(front XT) months. Marshy’s Method bleed plus levers rotated above/below bars while squeezing any bubbles out; cleaned excess fluid off calipers, applied silicone grease to pistons and worked them in and out a couple of times. Flawless since, no further cleaning or maintenance, better than new from CR. Going to repeat process at the end of the month, six month service time.