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Rebuilt Calipers: Tight Pistons


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Project:  Rebuild 4-Pot Calipers

  • bores and pistons polished; no rust anywhere
  • only gravity needed to insert and remove dry pistons 
  • new ring seals and dust boots
  • pistons, seals, and bore walls lubricated lightly only with BF
  • pistons now seated and fully retracted
  • no grease of any sort even close to the calipers
  • am well aware of need to insert pistons slowly, carefully, and directly straight into the bore


Issue:  Pistons were extremely difficult to insert.  Insertion felt as if pistons were having a difficult time passing thru the new seals (not the typical issue of being slightly off a straight path into the bore).  After struggling with #1, #2, and #3 pistons, discovered delicate use of channel locks helped a lot with #4.


Question:  How tight should the pistons be during insertion, past the seal, and after being seated?  Is really tight to be expected?  I assume pressured BF will have no trouble moving them during use; however, I am concerned about the ability of the seals to retract the pistons. 


Thanks, folks.




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Thanks, Marshall. 

     Yes, the pistons are really snug and tight now (likely b/c of the new seals?).  They went in smoothly, but slowly and with much effort.  This is my first caliper rebuild, so I was surprised at the effort it required.  

     Yes, pistons, walls, and seals were lightly lubed with only BF.



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     My understanding is, historically, yes, older seals might have been un-square in XS and needed to be installed a certain way to assure piston return.  However, I have found currently available seals are square in XS and do not need to be installed in a certain way/direction.


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I rebuilt my 528i fronts a few months ago the pistons were pretty snug so I opened the bleed ports.

With ample brake fluid for lube I used a Wetzler clamp and a wood cushion on the pistons but they went right in. Channel locks would also work but it defiantly was not a hand push in.

Most important they work great and retract correctly.

Edited by gary32
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when rebuilding calipers....after setting pistons in with new seals, i work the pistons in and out a few times before installing caliper in car.  compressed air and block of wood to extend piston, clamp/channel locks/brake pad spreader to contract (depending on the type of caliper)


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Thanks Marshall and Gary,

With piston #4 I discovered the wonderment of a piston-channel locks-wood combo.  Installation of pistons into caliper #2 should be easier.  The suggestion to work pistons in and out should be beneficial.

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I use something from PBR that I got from Pegasus racing for , because it's HARD to find brake lube that's

meant for the hydraulics.  They're now selling Mac Tilton's version for even more



but it helps a LOT more than using brake fluid, and doesn't promote bore rust outside the seals like

brake fluid does- and YES, a lot of the domestic lubes do-  Raybestos stuff's terrible for that.


And yes, if the pistons slide right in even with the lube, you have the wrong seals.  They should

drag pretty significantly on the 2002 and similar vintage calipers.  Square section seals, btw.




If you're in Oz, it's PBR RG-17.  If you're anywhere else, you get to order it on ePay FROM Oz.

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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