The International CBX Owners Association

So I rebuilt my entire front brake system. New pads, pistons, piston seals, brake lines etc. including the master cylinder. The front brake Calipers leak about a drip a day of brake fluid. The piston seals I used were Honda parts. The pistons I used were K&L from TIM'S CBX. I started filling them with a synthetic DOT 3 brake fluid with a mini-vac and later the old bleed method ( pump, hold, loosen bleed screw, tighten bleed screw, release lever ). After 4+ hrs. I got some resistance, but not what I wanted. The brake calipers ( both ) were leaking maybe a drip every 2 hrs. So I drained the system and filled it with some non-synthetic and after a half hour with the mini-vac I had more resistance than before. Still both calipers seem to drip. About a drip or two a day. What now? Could the Honda piston seals be as old as the ones I took out?

thomasgeorgi wrote:So I rebuilt my entire front brake system. New pads, pistons, piston seals, brake lines etc. including the master cylinder. The front brake Calipers leak about a drip a day of brake fluid. The piston seals I used were Honda parts. The pistons I used were K&L from TIM'S CBX. I started filling them with a synthetic DOT 3 brake fluid with a mini-vac and later the old bleed method ( pump, hold, loosen bleed screw, tighten bleed screw, release lever ). After 4+ hrs. I got some resistance, but not what I wanted. The brake calipers ( both ) were leaking maybe a drip every 2 hrs. So I drained the system and filled it with some non-synthetic and after a half hour with the mini-vac I had more resistance than before. Still both calipers seem to drip. About a drip or two a day. What now? Could the Honda piston seals be as old as the ones I took out?

It could be that things weren't cleaned good enough or perhaps since both are leaking that the pistons aren't proper size.
Seals, one side should be ridged and that's the side that should be facing the pistons.

I had a similar experience but had K&L piston seals, replaced them with Honda and the leak went away. I would check to make sure the seal groove is clean like Sasha suggests.

There are no ridges on the piston seals - the 'ridge' is in the caliper groove. We went through this just a month ago. Yes, I'd say you didn't get the caliper grooves completely clean. That said, you will get a bit of fluid appearing as a result of the installation procedure, but should clean off and be dry. Are you sure your leak is not from the banjo fittings?

For a painless bleeding procedure, search it up here - I've described it a bunch of times.

Dave

The thing is they weren't leaking before I changed the seals. I am somewhat sure that the pistons were the right size because I tried fitting them before I installed the seals and it was a really close fit. Zero room for slop. The pistons were hard to push past the seals when I did install them. I didn't see any ridges on the seals as I looked over them before I installed them. Maybe I should have looked harder. I am sure it isn't the banjo fittings, new seals all around and torqued to spec per the shop manual. Plus I looked several times. I read the bleeding section a month or so ago when I started this revamp; found it helpful. In the meantime the drip has slowed to a slight weep, and I hopeful. Thanks for your time and help. Also thank you for the post which mentioned that you need to pull the old spacer/frame from the front master cylinder cover and use it on the new cover. Caught me on that one. I thought that just the rubber diaphragm and cover was enough.

As that rubber piston seal sits in the caliper bore, inevitably some fluid gets behind it as it flexes in operation, dries, and creates a layer of crud, mostly dried brake fluid. When you rebuild, you can sit there with a pick and solvents and what have you and think you cleaned it spotless, but I'm here to tell you that you did not, and that layer of crud is pushing that nice new seal in tighter than ideal contact with the piston. Take a Dremel with a brass tipped rotary brush and ring around that seal groove with it, being careful to get the brush tips into both corners. You will see a *poof* of dried brake fluid when you do this. Follow that with a clean in some nice soapy water, dry, reassemble with a smear of silicone brake grease on them and the piston and that's the best you can do IMO.

Thanks Nils. I'll be doing that next but it may take some time as my progress keeps getting interrupted. I'll get back to let you know what happened.

One other 'Tip' for cleaning brake calipers: Remove everything from the caliper and submerge it into a pot of boiling water for 10 or 15 minutes. Take it out, shake off excess water and let it air dry. Turns most/all the brake fluid crap into a dry white powdery substance that is much easier to remove/clean from all the grooves, crevices, etc. The brass brush/Dremel that Nils mentioned will, then, have an easy 'job'' removing the crud.

Part 2: So I disassembled all the calipers. Boiled them in water for at least fifteen minutes each. Let them dry, then made three light passes around the piston seal groove with a brass wire brush in my new Dremel; and then made three light passes with the small cloth mop also. Then washed them out. I noticed that the cross-section of the piston seal is more like a trapezoid than square, with the high point about 0.2 mm toward the caliper casting and the part toward the piston is square. When I assembled it again I put that high point toward the outside of the caliper. It was easier pushing the piston in with my thumbs. Much better results bleeding the lines and after four days no leakage. Thanks for the help Nils and Larry.

You can really go after the bores with the wire brush and Dremmel. It takes VERY firm pressure to get all the crud. But you must have gotten it OK. But once again - the caliper rubber rings are SQUARE (or maybe rectangular) - period. If they have been in a caliper, they will have assumed the tapered shape of the groove, but it DOES NOT matter how they go back in. They will immediately assume the taper, or step, of the groove they are in.

Glad you got it. Now you know what to do the next time!

Dave

Remember the old can openers witht the pointy triangular end? The kind that opened a triangle in the can? Those work very well as the first step in cleaning that groove in the caliper, then proceed with the brass brush for the remnants.

Yes, I remember opening beer cans with those openers........

Way to go Thomas!

Great! I luv happy endings!