Brake piston opinions?

Hi all,

I'm doing a full rebuild of my front brakes on my XJ550 4V8 (European model with twin front brakes).

The repair kit for the calipers i have didn't include new pistons and I would like your opinion on the condition of the old ones.

The worst one first:


And than the second, better one:


What you see is the absolute worst parts of them (not counting the inside), and there are no other major marks on them. Both go very smoothly into the caliper housing.

Would you reuse them or just toss them and buy new ones? Money is an issue right now, so it might have to wait for a couple of weeks if I have to buy new ones.

A third option is to go with the old ones for now and exchange them during the winter.

Since I have two front brakes and a soon-to-be-rebuilt rear brake, I'm not as worried about stopping power as of one of them locking the front wheel. Could the damage you see cause that?

Lastly, below is also a picture of all the parts layed out.

 

Its good to see someone do a proper brake rebuild of every component.
The first piston looks borderline if the pitted area contacts the main piston seal it will leak. It does look like it will,. The second piston looks OK.

 

I'd agree - that first pic is borderline. However, as long as that pitted area remains behind the main seal it won't pose a problem. If/when the piston extends (due to pad wear) and that pitting reaches the main seal it would surely impact the integrity of the seal.

I'd be willing to bet Chacal has the pistons - seeing you are doing a good full rebuild you might want to do it now and be done with it.

 

Actually, it's more that the pitting is already to the outside of the seal area; but is it completely out of the area swept by the seals?

The pitting on the first one does look as though it goes far enough to be into the area swept by the main seal. If it is, it will leak. Drop it into a seal-less caliper body and see where the pits line up.

I would replace #1 and reuse #2.

I would also replace, rather than polish and re-use, the "sliding pins" that hold the caliper and holding block together.

Chacal (XJ4Ever) also now has the caliper pin BOOTS (the ones on the holding block) in stock; Yamaha never even gave them a part number.

Also: I don't see any brake pad shims in the pic. The pic below was for somebody who needed to fab one; chacal now carries them though. They go on the piston-side pad.

 

Thanks for the answers!

I will check how deep the piston pitting goes inside the caliper when I get home tonight.

There are no brake pad shims on any of the brakes, but I don't think it's the same brake that is shown in the exploded diagram. My dust covers and shim plates looks a bit different than the drawing (though the larger shim plate is partly hidden in the drawing. I've read here on XJBikes that there are several different caliper versions, sometimes differing from year to year. Mine is a European XJ550 4V8 model from 1982. Do you know if there is ANY caliper model that doesn't have this brake pad shim? The piston pushes the brake pad directly and this is how it's been since I bought it. Granted, I am the 13th owner and have no way of knowing if any of the 12 POs removed the shims for some reason.

The sliding pins have been thoroughly brushed, sanded (up to 400 grit), polished and goes through the caliper and holding blocks very smoothly now.

I have the caliper pin boots but I forgot to add them in the picture. Or rather, I have 3 out of 4, as the last one was broken. I'm thinking about fabbing my own as I have a lot of rubber parts from other projects, some with a perfect fit around the caliper pin. I have an idea combining these with shright tube that should allow the pins to float as with the original boots. When checking the part list from chacal (XJ MAXIM, SECA, AND TURBO PARTS CATALOG), there is no mention of these boots for the XJ550, only for the XJ650 Turbo and XJ750 Maxim and XJ750 Midnight Maxim models (or did I misunderstand it?).

I will probably place an order from Chacal later when I can afford it, but right now I can't afford spending $181 + shipping to Sweden + 25% import tax for two pistons and 4 rubber boots. At an estimated shipping cost of $15 and the average boot price of $14, the total sum is about $240. That's more than 50% of what I paid for the entire bike.

I know I have to buy these parts sooner or later, but right now I really can't. Does anyone know of a European vendor that carries these parts cheaper?

 

In answer to my own question, ordering the same parts from Yambits only cost the equivalent of $95 for 2 pistons and 4 rubber boots, including shipping.
Ordering only 1 piston and 2 boots (since 2-3 are still in fair condition), the price drops to about $50. I can handle that. Still, if anyone knows an even better price, please let me know.

 

go for it!

 

Ok, I've just measured. The pitting does not touch the main seal even when the piston is in it's very bottom position, except for two tiny spots >1mm. If the brake pads have a distance between them of 7mm, no pitting whatsoever touches the main seal.

The pitting does however go along the dust seal in the bottom position.

Is it foolish to keep using it?

 

*NOTE: the main seal is the one furthest into the caliper bore - the dust seal is the outer one closest to the pads! It is the main seal that you should be most concerned with (the one that contacts the piston farthest from the pads)

The way your post read I believe you are measuring against the dust seal (unless I read it wrong). Be sure you are measuring against the main seal.

 

Just re-read... I think I did indeed read it wrong. If the pitting on the piston, when bottomed out in the caliper bore, does not reach the main seal then IMHO I think you'll be fine. The one piston whose pitting goes across the dust seal will be more prone to letting crap past the dust seal but I've found that most of the "crap collection" occurs behind the main seal.

If money and availability is not an obstable - I still vote replace the sucker and be done with it. If it is a obstacle then I believe you have nothing functional to be concerned with - especially if you plan on replacing that piston at some point down the road.

 

Were they leaking before dissambly?

 

Correct, the pitting only reaches the dust seal. I noticed that when having the seals in place the piston doesn't reach as deep in the caliper, so the affected area is even smaller. But maybe this is just an optical illusion since the seals forces the piston to be centered in the hole, not making the gap where you could see the pitting as noticeable.

Yep, I reused it.

This is one of those months when all bi-monthly, quarterly and yearly bills all came at once. And I unexpectedly had to go to the doctor a couple of times as well (other story). I ordered the lines and rebuild kits before the economical shitstorm started.

I have just been bleeding the brakes, taken a trip around the block and then bled some more. Will do it a third time after riding a few kilometers, hopefully tomorrow. Brand new organic brake pads as well, which requires that you do not do any hard brakes the first 100 km (about 60 miles) or so.

Does that sound like the right procedure? First time I've ever had to bleed brakes. Aside from new brake pads, I also have brand new stainless lines; 2x custom-length (130cm), DOT-approved brake line assemblies from HEL including all bolts and washers. About $80 total including shipping from the UK.

Nope. I've put on ape hangers and just needed longer brake lines. But after reading a bunch of threads here on XJ Bikes, the project kind of grew of itself. I actually felt kind of peer pressured to do a complete rebuild of both MC and calipers, just reading other threads. :roll:

Damn you, XJ Bikes... damn you!!
/The Wallet

Oh well, I guess it's best to have it done in the end.
Especially to know that it's done good and well. The calipers were in pretty bad shape and might have caused problems further down the road.. or eventually off the road.

 

With the work that it looks you put into it you should have good firm brakes with no pad drag. Sounds like its time to put some wear on those pads now!