Curing a brake caliper leak

[Stevex]

I have a Brembo twin bleed P08 caliper fitted to rear of my LM2. After 3 re seals it still leaks fluid.
The leak is coming from the piston / caliper interface. As well as replacing the seals that sit in the caliper halves, I replaced
the pistons as well with no difference. I have been told by a supplier of refurb'd Brembo calipers in the UK, that some of these calipers
leak around seal where they sit in the caliper halves, due to porosity.
Any one heard of or suffered from this and if so, is there a cure?
I've toyed with the idea of buying a new single bleed P08, but it seems a pity not to be able to use the now rare and original twin bleed.

[canuck750]

I have a leaky P8 dual bleeder in the Laverda SF1 I restored.
I think it may be the O ring seal but I need to tear it apart and see.
Like yourself I have replaced the seals, pistons, pins, pads and installed speed bleeders.
This is the first one I have had that leaks having rebuilt at least a half dozen or more of these dual bleeders.

[rodekyll]

In what condition are the piston bores?

Porosity sounds like nonsense to me.  If it was porous it would have been leaking long ago.

[tris]

If it is porosity see if you can track down a good casting company that do valves or something similar that's pressurised as they often resin impregnate the casting from scratch.
If they don't do it they might be able to point you at somebody who can

[nc43bsa]

Is the o-ring groove in the caliper half clean and smooth?

I typically scrub them with a Dremel, nylon brush and some aluminum polishing past or toothpaste.

[canuck750]

This is a picture of one of my Laverda P8 calipers during rebuild, the castings were spotless, glass bead and then ultrasonic cleaning. Still leaking!

The casting with the shallow recess for the O ring which seals the transfer port is good, the other smooth face ain't perfect, maybe I need to swap out the smooth face casting or I could try and surface grind the casting.

Any thoughts from you machinists on setting up the casting in mu small milling machine and running a fly cutter across the surface?

[Stevex]

Having had the caliper apart 3 times I've obviously inspected the caliper bores and seal recesses thoroughly and they appear in excellent condition.
I too doubt the porosity theory, at least with my caliper.
The small seal at the transfer port is definitely not the culprit; it's not until I peel the piston dust covers back that the leak is obvious.
I had the caliper coated but the piston bores were blanked off and the stainless pistons were replaced after the first leak, with alloy ones, even though they too were in excellent condition.

[RinkRat II]

 Couple of questions: what is the bore to piston clearance,and are the seals a quad type ring or standard o-ring?


   Paul B

[daytonars97]

This is a picture of one of my Laverda P8 calipers during rebuild, the castings were spotless, glass bead and then ultrasonic cleaning. Still leaking!

The casting with the shallow recess for the O ring which seals the transfer port is good, the other smooth face ain't perfect, maybe I need to swap out the smooth face casting or I could try and surface grind the casting.

Any thoughts from you machinists on setting up the casting in mu small milling machine and running a fly cutter across the surface?

The problem with leaking of these calipers on rebuild may be with the small O ring shown in the upper right quadrant of the caliper half in the pic.  IIRC the rebuild kit does not include that "o" ring.

[canuck750]

The problem with leaking of these calipers on rebuild may be with the small O ring shown in the upper right quadrant of the caliper half in the pic.  IIRC the rebuild kit does not include that "o" ring.

The rebuild kit does include the transfer port O ring.

[daytonars97]

The rebuild kit does include the transfer port O ring.

Good to know, Canuck. Where did you buy yours?

[RinkRat II]

Hi Canuck, If I may suggest before milling that surface,to lap it on a sheet of glass with some 120grit wet or dry and some solvent as lubricant and I would think that area will clean right up.
 
  Paul B

[Lumpy Idle]

i second rinkrats's idea about lapping it yourself. clean the surfaces with alcohol and 'blue' them with a fat sharpee. then check as you go. ciruclar motions. when all of the sharpee 'bluing' is gone - yur dun.

[Muzz]

Are you using silicone brake fluid?  That stuff is a real swine to get to seal.

[canuck750]

Hi Canuck, If I may suggest before milling that surface,to lap it on a sheet of glass with some 120grit wet or dry and some solvent as lubricant and I would think that area will clean right up.
 
  Paul B

I like that idea! I would probably just bugger it up on my mill.

I got my rebuild kits from MG Cycle.

I need to read the label on the brake fluid I have used, nothing special if I recall correctly. I think it was DOT4, does that sound right?

[Muzz]

 I think it was DOT4, does that sound right?

I think the silicone is 5.1. Dot4 sounds good.

[tris]

I was looking for something to stop the balance screw on by B11 moving from where I set it and discovered Loctite 290 - wicking grade

http://na.henkel-adhesives.com/us/content_data/316041_FINAL_LT4680_AAM_Do_It_Guide_v7.pdf

I accept that the problem here may well not be porosity, but if you go to page 12 on the above you can see how 290 might be a solution if it was