Caliper rebuild problems

[FrkyMnky]

I rebuilt both front and rear calipers with replacement mikes pistons and seal kit. For whatever reason, the rear would not build pressure at all, so I pulled it off and noticed the piston was being pulled back into the caliper. Pulled off the dust seal and noticed when I pressed on the master cylinder there were tiny air bubbles coming out from around the piston seal. Thought there might be a problem with the mikes kit, so I bought a Yamaha rebuild kit and replaced the seal. Same thing, couldn't build up pressure. I pulled off the front caliper as I wanted to check that as that one was working, and when I pulled off the dust seal there was brake fluid underneath. Pulled the front brake lever and I had the same thing happening, air bubbles coming out from around the piston.

When I assembled the calipers, I used brake fluid for everything and coated the seal. The Yamaha kit I received had grease in the kit which I am assuming was to coat the seal and piston. I didn't use it and simply used brake fluid. What in the world is wrong with my calipers?

 

[2eightcustoms]

is the piston pitted at all?

 

[FrkyMnky]

Nope, brand new pistons in both.

 

[Brian902]

Any chance the leak is between the seal and the caliper and not the seal and the piston.

The square groove the seal sits in must be clean and free of corrosion layers that often build up there.

Magnifying glass and re-check the seals for cracks due to age, but that seems unlikely with new parts.

Pistons must be absolutely smooth of course.

When bleeding, compress your pistons into the caliper as far as possible and use a clamp to hold it there. Then start bleeding. Less space for air in the caliper makes bleeding faster.

How tight do the seals fit into the caliper and how hard is it to get the piston in? Any chance they are the wrong size? Measure old or original parts to new parts.

I do like you, just use brake fluid for lubrication of parts before install. No grease.

 

[FrkyMnky]

I pulled the calipers apart today, and honestly I have no idea. The calipers were just sand blasted and everything was cleaned out. There was nothing inside the bores or the seal groove. One of the seals is brand new and the other one is in great shape as it was replaced just over a year ago.
I even cleaned everything and tried assembling everything dry to see if that would change anything and no joy. I put the seals around just the pistons out of the caliper and there is a small gap all the way around. I expected it to be tight around the piston, but they are not. At this point, I am really lost.

 

[Brian902]

Seals should be tight when in the caliper and installing the piston.

Not sure if I've ever tried to install a seal over the piston as a test.

I would think it should be snug, not tight but definitely not too loose.

Remember it's got to make that perfect squeeze so I'm thinking yours are too big.

The rubber deforms easily when hit with parts cleaning fluid and other chemicals.

Perhaps Yamaha sells just the seals, or if someone on this site happens to have a brake job in progress they could give you some more information.

New seal kit to test would be ideal.

Just re-read your post, small gap, I'm thinking it should be mighty small.

 

[FrkyMnky]

Well at least one of the seals is brand new, and the other 2 are lightly used. They all measure the same, and once in the caliper it is a tight fit. I am figuring the only thing that could be messed up at all is the pistons. So I can hope that is what is wrong and that nothing is wrong with the calipers and I can simply buy new pistons. Or, I can simply buy new calipers at 4x the cost. I hate not knowing exactly what the problem is.

 

[Brian902]

Yes this would be annoying, you've done everything right and still bad results.

Mikes XS shows dimensions, do these match the old pistons?

The only other thing is the brake fluid you are using.

New bottle of Dot 3 would be recommended.

I've used silicone brake fluid Dot 5 in mine.

Given everything you told us it should be working. New pistons with smooth surface and new seals with new brake fluid.

I know bleeding and building pressure in the system can be difficult but starting at the master cylinder and bleeding the joints on down always worked for me.

Perhaps another set of eyes could spot the issue. Any biker friends around you with mechanical experience.

Good Luck!