Topic: brake pistons - pitting in the seal groove

[superchode]

after sanding the rust off of my brake pistons - some fairly serious pitting has revealed itself in the seal grooves of two of the three ('78 750F).

can i fill these pits with JB Weld or similar? I'm tending to think it'll leak if i drop a new seal in these pitted grooves.

anyone have experience with this?

[Tim.]

Replace the piston if it's that bad.  You can't correct this with JB Weld.

[superchode]

three new pistons was an expense i wasn't bargaining on...

while i'm on the subject - one of the front pistons was longer than the other one (the short one appeared to be the same size as the rear piston). the calipers appeared identical... what's with the extra length piston, is it just the geometry up front?

[endobendo]

Hey choad,

can't comment on the piston design but I do share the pits with you.  My rear pistons on my 75 750F were pitted so bad, they tore up new seals and wouldn't hold any fluid.

I found that the rear pistons are no longer available.  So I had a local machinist make me two out of stainless steel.  I'm going to put them in as soon as I get my new seals.  They look identical to the original ones and all I had to do was roto till his garden, change the oil in his windmill and buy a bottle of crown royal.

It pays to have semi-retired machinist farmer types around.

[scondon]

after sanding the rust off of my brake pistons - some fairly serious pitting has revealed itself in the seal grooves of two of the three ('78 750F).

    Just so I have this straight, you are talking about the groove that the dust seal clips to at the top of the piston? If so, I would be of the opinion that attempting a JB Weld fix would be OK as this area does not enter the bore of the brake caliper.

    One of my '78 750's was missing two dust seals when I bought it. Moisture had entered and pitted the pistons but there was not a lot of dirt in there and the fluid seals were not leaking. Not sure why I'm mentioning this

[Noel]

New pistons would be my automatic response, but the more I think about the JBweld trick, the more I think it might work.

Seems to me that if the JBwelded pistons are carefully smoothed and polished after drying there should be no problem. Assuming, that is, that JBweld doesn't expand or weaken in contact with brake fluid.

I think it's worth a try. If it works, it works, and if it doesn't, the worst possible outcome is probably a system that needs to be completely flushed to get rid of hunks of JBweld, and then fitted with new pistons.

[Bob Wessner]

If it works, it works, and if it doesn't, the worst possible outcome is probably a system that needs to be completely flushed to get rid of hunks of JBweld, and then fitted with new pistons.

Maybe one way to conduct such a test so it won't cause any adverse problems is mix a small amount of JBWeld and when cured, drop it in a small container of brake fluid and observe any changes over a period of days. Just a thought.

[mick750F]

three new pistons was an expense i wasn't bargaining on...

while i'm on the subject - one of the front pistons was longer than the other one (the short one appeared to be the same size as the rear piston). the calipers appeared identical... what's with the extra length piston, is it just the geometry up front?

   All three pistons should be the same as they have the same part #. I'd definitely go with new pistons if there is pitting in the seal groove. Someone was selling replacement pistons on the site a while ago. If you do a search you might be able to find them.

Mike