How do I rebuild an Avid J5/J7 caliper the right way?
I rebuilt a 2007 J5 rear caliper about six month ago because the inside piston was sticking more matter how much mineral oil or DOT brake fluid I dripped on its seal to break it free. I rebuilt the caliper with a new Avid seal and piston kit and used DOT fluid on the inside lip of the seal before I pressed the piston into its bore. The new inside piston moved freely in its new seal for about two months after which it started to stick again. I tried all of the usual remedies to free the piston but unfortunately it finally froze in its seal after a few more rides.
Luckily, my LBS called Avid and the both the front and rear 2007 J5ís were warrantee out for a set of 2008 or 2009 J7s.
The new J7ís have worked flawlessly for about six months until I noticed both the front and rear pads rubbing on their rotors. When I tried aligning the calipers without success, I knew the pistons were starting to seize up. After about spending an hour repeatedly cleaning the pistons and dripping DOT fluid on their seals I freed up the pistons in the rear caliper. Unfortunately I cannot free up the pistons in the front caliper so it will need to be rebuilt.
So why do my Avid pistons keep seizing up? Is the rubber compound in the seals reacting poorly to type of trail soil and road base I ride through? Or do I need to use a special lubricant on the seals before I press in the pistons? When I rebuilt the J5 caliper, the old seals were much softer (sponge like) compare to the new replacement seals.
"...the inside piston was sticking more matter how much mineral oil or DOT brake fluid I dripped on its seal..."
How often are you applying mineral oil to your seals? Given that the reaction of silicone seals to mineral oil is that they will swell, this may be part of your problem.
New seals will be harder as they're dry. They will aborb a small amount of brake fluid when they are put to use and this will help them to maintain a (hydraulic) seal and lubricate the friction between themselves and the pistons.
You'll be able to get a silicone grease for fitting new seals, but in my experience a simple coating of DOT fluid is sufficient.
I think that my suggestion would be to get into preventative maintenance. If you've been out for a ride in the wet, or after every two or three dry rides, just whip the pads out and spray some silicone spray into the caliper interior, wipe the exposed pistons with a cotton bud, then thoroughly dry off the residue with a cloth/tissue. This process only takes five minutes per brake and can save you a lot of messing around further down the line. It's at least worth a try to see if it makes any difference.
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According to my LBS mechanic, non-silicone based DOT fluid and or mineral oil are the only mediums that Avid recommends to free up their caliper pistons. I believe DOT 2, 3, 4 and 5.1 brake fluids are all mineral oil based. 5.0 DOT brake fluid on the other hand is silicone based and cannot be used in mineral oil based hydraulic systems because it will quickly break down the seals and create a leak.
If you read through the Turkey Warbler post, many owners of Avid hydraulic brakes have sticky pistons, which manifest into vibration. I found this out (i.e. stumbled into this discovery half backwards) after I rebuilt my J5 caliper. This 2007 OEM J5 caliper vibrated the back end of my mountain bike violently plus warbled a week after I purchased the new bike. The vibration-warbler occurs from one piston-pad pushing on the rotor from one side only. Because the opposing pad cannot be pushed into the rotor, the rotor will deflect from the brake pad with the active piston side until it is stopped by the inactive brake pad-frozen piston. The rotor flexes enough so that the diagonal slots that are punched in the G1, G2 and G3 rotors catch on the edge of the brake pads to create the vibration. You can recreate this vibration at a slightly different frequency by setting up an Avid BB mechanical caliper incorrectly. For example instead of dialing in a small gap between the stationary pad and the rotor, dial in a large gap and Bingo, you created a warbler. Likewise if your Juicy caliper is vibrating dial in a very small gap between the pad with the sticky piston and you fixed your warble. Unfortunately, your brake will suffer from diminished clamping forceóbraking power.
I know both Hayes and Shimano had problems with their pistons sticking in the past so I am not sure if their Engineerís figured out a solution yet. Certainly Avid is going through a learning curve with the caliper piston seals. What we need is an automotive brake company, who has perfected the seal compound, to sell seal kits for Avid brakes.
No! Like Steve said, don't use mineral oil around those brakes!
Originally Posted by bwalton
quoted from wikipedia: DOT 4, like DOT 3 and DOT 5.1, is a polyethylene glycol-based fluid (contrasted with DOT 5 which is silicone-based).
Is there a trick that will keep my pistons from sticking?
SteveUK is absolutely right about not using mineral oil on a seal designed for DOT fluid (glycol ether/borate ester) compound. I wonder why Avid suggested mineral oil. Likewise, you should restrain yourself from using a silicone based lubricant on a DOT fluid compatible seal as well. Brake fluid is really the only compatible medium that works on Avid seals.
Unfortunately, I could not free up the pistons after applying many drops of DOT fluid in the piston bores, moving the pistons out by actuating the MC lever and pressing them back in. I spent a good hour tinkering with this problem before I bled the brake, which had no effect on the pistons either.
I found a few articles about sticky pistons in motorcycle calipers. They attributed the problem to a build up of oxidation in the seals metal seat-channel, which distorts the seal enough to wedge up against the side of the piston. The articles recommended removing the oxidation from the seat with a soft tool, remove any small pits from the piston with very fine sand paper, remove the layer of varnish from the seal with your finger nail and reassemble the caliper.
When I rebuilt my J5, the sticky piston had wear lines around the sides (like a stripped shirt), no visible oxidation or debris in the seat or any varnish on the seal. The inside cylinder did have some metal shavings in it that were not cleaned out after the factory drilled the port. As mentioned in my beginning post, the new seals and pistons worked for only two months before they started to stick again. So either the pistons or the seal seats are not machined round, have poor fit-up or the rubber compound in the seal is not formulated well. Another possibility is that Avidís brake fluid has lubricants that break down very quickly. The absorbed water in brake fluid not only lowers its boiling point but it also degrades the lubricant as well. Maybe, the absorbed brake fluid in the seals is so full of moisture that the lubricant had dried up.
"Likewise, you should restrain yourself from using a silicone based lubricant on a DOT fluid compatible seal as well. Brake fluid is really the only compatible medium that works on Avid seals."
Silicone spray is fine because piston seals are made of silicone rubber. I've been using it on every brake I've worked on in for last four years or so, with consistently successful results. I started to use it on the recommendation of one of the guys at Hope.
What use is a philosopher who doesn't hurt anybody's feelings? -