Front caliper overhaul

[John_Mc]

Adding to my suspension project, I thought I'd look at the front calipers before I reinstalled them. The rubber looked iffy, two of the circlips holding the rubber on were loose, and the exposed surface of the piston (once rubber was removed) had a bit of rust on it. How hard is it to just buy the components (ie new pistons and rubbers) and reinstall them? I was able to get one piston out using air pressure, but once it's out, how do I get the second one now that the system is open? If I ever get this front end back together on my own, I'll be amazed.

 

[Banjo]

JOhn,
Parts are readily available through the usual suspects. Moss, Victioia British ETC... What you wanna do is refit that piston just enough to get a seal, then hold it with a pair of channel locks, or a "C" clamp and use the air to blow out the other piston.
If there is ANY rust on the pistons, replace them. It's not worth the safety risk to try and clean up the old pistons.
Good luck Guv'na

 

[Guest]

Hey John, I cheated. I sent mine off to Henry Schein, oops, TRF, as cores and had their newly overhauled ones to put in.



<span style='font-size: 8pt'>Inside dentists' joke.</span>

 

[John_Mc]

Thanks Banjo. Duh, I should have thought of that! So I took my calipers to work with me and had a break between patients to sneak back to my lab and blow the pistons out with some compressed air. Voila! Luckily my assistant was observant enough to notice the brake fluid on my mask....hopefully before my patient did!

I'm really considering cheating Bill! But I also enjoy the satisfaction of cleaning things up myself when I can. I just need to know when I'm fighting a lost cause or not. I have new pistons and rubber parts on order from BPNorthwest. There is some "residue" in the piston bore of the caliper. I thought about pulling the two halves of the caliper apart but I'm not sure I want to add yet another "pull apart-put together" to this front-end rebuild. Can I smooth this out with a light touch of emory cloth or similar?

 

[70herald]

There is some "residue" in the piston bore of the caliper. I thought about pulling the two halves of the caliper apart but I'm not sure I want to add yet another "pull apart-put together" to this front-end rebuild. Can I smooth this out with a light touch of emory cloth or similar?

Just smooth it out. The cylinder is not a precision or critical surface in this brake design. The Piston on the other hand is critical. just clean it out as best you can.
The o-ring between the two calipers is also a square profile. If you take the calipers apart make sure you can order a new -ring before you take it apart!

 

[tr6_easyrider]

70herald, I thought the cylinder in the caliper is the critical surface.
The o-ring on the piston makes it's seal with the cylinder suface. Am I reading your post wrong?

 

[DrEntropy]

No, he's correct: The piston-to-seal is the critical bit. Seals are in the caliper body in a recessed square-cut lip, not on the piston. The caliper bore is just a guiding surface/ pressure chamber. Pistons need to be absolutely smooth/clean to maintain the seal. If pitted or rough the seal is abraded and leaks. It's why pistons are chromed or made of stainless. /bcforum/images/%%GRAEMLIN_URL%%/thumbsup.gif

 

[70herald]

70herald, I thought the cylinder in the caliper is the critical surface.
The o-ring on the piston makes it's seal with the cylinder suface. Am I reading your post wrong?

Just to make sure we have the terminology correct here:

The caliper contains the cylinder. The cylinder has a groove cut into which the square shaped o-ring fits into. The piston is the metal cup shaped object which moves in and out.

The only surfaces on the caliper which is critical is the o-ring groove which needs to be nice and clean. The outside of the piston is critical since this is where the "action" takes place.

Note that the inside of the caliper (the cylinder or bore) is not polished. The moving part: the piston is polished on the outside where it seals against the o-ring. Since it moves, if there was damage or scratches on this surface it would damage the o-ring. Even if there is some sort of scratch on the o-ring grove, the o-ring never moves relative to the grove so it wont hurt anything. The inside of the caliper (cylinder bore) only touches the brake fluid and as long as it is clean it is good enough, no need to polish, hone etc.

I pointed this out because it is the opposite of the hydraulic cylinders used in drum brakes, master cylinders and the like where the Cylinder wall is critical (the sealing surface) and the piston is not (except for the small ridge which holds the o-ring in place.