Topic: Brake Caliper Rebuild Issues

[Vintageguy]

1978 750F. Per advice on this forum I attempted a rear caliper rebuild today on my beloved Super Sport. I got everything immaculate, and I do mean absolutely spotless. Installed a new dust seal and main seal with clip. When I re-installed the piston it took quite a bit of pressure to get it in the bore. More than I had anticipated. Needed a c clamp to install it. At any rate, I can't seem to bleed out the caliper now, and it leaks fluid from the bore. Took my time, Got everything extremely clean as mentioned. Where do you think I went wrong? Confused and pissed. 

[Doobie]

Did you lube the piston and gasket before reassembling? There is no way you would have needed a clamp.

[Vintageguy]

Did not lube.

[Flyin900]

The slot that the seal sit in must have nothing in any nooks or crannies and you say it was spotless, yet if the seal is seated in a spotless bore you can push the piston in by hand fairly easily.
I would take it apart again and review that area since it is leaking there appears to be something not right in there.

OEM Honda seals have a lube on them from the factory aftermarket not so and if aftermarket there could be an issue with the seal out of spec if a cheap set.

[BenelliSEI]

As previously stated, you should be able to push the piston in with your fingers. Use a drop of brake fluid on the seal fro lubricant. Maybe the dry seal “rolled” in the grove?

Either that or the grove has built up crust in the back, or the seal is the wrong size…..

[bryanj]

If fluid is leaking out the bore then you damaged the seal fitting the piston, pump out the piston with a grease gun then reclean the groove and fit new seal again, lube with brake fluid and as stated piston should push in with finger pressure

[Vintageguy]

My bad. I debated if to lube or not. Sure will next time. Does anyone know where to get a genuine Honda seal set? Thanks for the responses.

[Don R]

https://www.partzilla.com/catalog/honda/motorcycle/1978/cb750f-a-750-super-sport/rear-brake-caliper

[bryanj]

You wont get a SET honda sell the seals seperately, see the parts list posted

[PeWe]

Good to lube all seals before pushing them on or in something. Engine seals too. Seals that get friction.

Not o-rings that seal facially as head, cylinder etc.

I have lubed the fork seals with fork oil. Even added some oil on the upper side to lube them, reduce friction and wear.

One thing is clear, do not use silicone grease to lube caliper seal. I made that mistake when I added an extra rotor at front on my K6.
Brake fluid really flowed all over both brakes after a very short ride.
Take it apart, cleaned with acetone and new seals.
Used brake fluid for a smioth marriage, piston-caliper. I could reuse the new pads.

I think the seal had to be flipped correctly too, one side is different if I remember correctly. I guess that can be common for piston retraction to work.

I have some extra seals after that. Added when ordering other parts. Those were very cheap at Yamiya several years ago

[bryanj]

Seals are square section, it is the groove that is shaped  to turn the cross section diamond shaped with one corner rubbing on the piston side.
As the piston moves through the rubber the rubber distorts and twists and it is this twist that pulls the piston back from the pad

[PeWe]

OK, good to be reminded and correct info here.

[Johnie]

And make sure that piston has no rust or sharp pits that will score the seal.