1982 LS Brembo caliper size?

[R65Singh]

Hi Fellow r65 riders,  I was going to order the rebuild kit (pressure seals) for my bike and when I searched on the net it says it is 38mm, but when I actually measured the piston size it is 36mm.  The cylinder is also about 36mm.
To add further to the confusion the the pressure seal that came out is in good shape but about 2mm loose on the piston.  Is this normal or someone installed wrong pressure seals.  The brakes always seems to be dragging no matter what I did and finally I took them apart to take a look.
I was going to order these from Ebay.

http://www.ebay.com/itm/36mm-OES-Generic-Brembo-Brake-Caliper-Individual-Pressure-Seal-Dust-boot-2-each-/281233166455?pt=Motors_Car_Truck_Parts_Accessories&hash=item417acd9077

What you guys suggest?  Thanks!!

[Barry]

36mm is the correct size for an R65.

The seal does look too big although they are not usually a tight fit on the piston before being installed in the caliper. I suppose it could be the wrong seal or it could be swollen. What counts is how it fits in the caliper. Does it go in evenly without rucking up and then how tight a fit is the piston when pushed through the seal.

This is a tough one to call as the piston is normally a relatively stiff fit in an installed seal but it should not be so stiff that it doesn't retract slightly when brake pressure is released.

The top picture in the diagram shows that the seal is supposed to distort when the brakes are applied pushing the piston out to the right. Then in the bottom picture the tension in the seal pulls the piston back a few thou to the left when the brake is released. That pull back doesn't happen if either there is too much stiction due to corrosion or if the seal is too tight.

When the brakes are assembled you can actually just about see that movement with the naked eye or in my case with my best reading glasses. If you don't see that movement with clean pistons then new seals are going to be needed.

[montmil]

Clean the caliper bores and completely flush the brake fluid prior to reassembly. Check the master cylinder and swab out any goop. Check the tiny return hole in the MC's piston as it should be clear. Thin strand of copper wire used as a probe.

As brake fluid is hygroscopic, it attracts moisture and can lead to corrosion within the caliper/s. Polish the pistons. Oft times there may be a bit of rust above the square-section o-ring but than can be smoothed away with some 400 Wet-Dri paper and should not affect brake performance. Clean, clean, clean. Lubricate the parts with fresh brake fluid during refit.

May as well run the brake pads over some 150-ish grit to clear any glazing. A quick, figure-eight sanding motion will prevent grinding an unwanted taper on the pads. Clean, clean, clean.

[Bob_Roller]

Brembo pistons should be aluminum, not plated steel, I would not use sandpaper on them .

They have an anodized surface I wouldn't us an abrasive on them .

[R65Singh]

Thanks for your help guys.  
Monty, every thing is very very clean and brake fluid is new.  There was absolutely no rust on any of the parts and pistons seems to be coated with some sort of Teflon so i just cleaned them with soft cloth. I put it back using the old seals as I don't know which seals to order. The seals go in very nicely without any rucking up.  Everything fits back smoothly and looks like the seals are correct size even though they are awfully loose around the pistons. MC is nice and clean with no gunk or rust and all the ports open.

The seal size question still remains whether it is 36m or 38mm, although seals looked more closer to 38mm, even though piston is 36mm.  May be they were 36mm but swollen up after use?  Would be nice to know what size to order for future use.

The wheel is much easier to rotate but I can still hear pads rubbing on one side, although it is much less than before. I can see the piston moving back and forth only on one side of the calipers.  Pistons moves much more freely on the side where the brake hose connects.

For those who have never done it, it is much easier job than i had thought.  Those seals come out very easy using a dental pick. You do have to watch, which way they go in.  Pictures provided by Barry are a great help in understanding how they work.  

[Barry]

The wheel is much easier to rotate but I can still hear pads rubbing on one side, although it is much less than before. I can see the piston moving back and forth only on one side of the calipers.Pistons moves much more freely on the side where the brake hose connects.

Sounds to me like you are pretty much there or thereabouts.

Drag is never completed eliminated as even if the pistons move back it doesn't drag the disc pads with them. They are pushed back by the discs so they must be free to slide in the caliper. If I spin my wheel by hand and it rotates 2 or 3 full revolutions before coming to stop I consider that good. You can get it perfect but there is no use pretending that will last from one brake service to another.

Having one piston move more easily than another is almost inevitable. What you could try is to remove the pads and hold back the piston that is moving freely with a G cramp and then exercise the the other piston through a few cycles by applying the brakes to force it out (not too far) and then push it back in again with another G cramp.

[montmil]

Brembo pistons should be aluminum, not plated steel, I would not use sandpaper on them .

Bob, I was referencing the area on the piston that's up by the dust seal. The actual piston in bore ought to be smooth and without damage.