@texases thanks, I have a lot of questions, but I’ll have to do some work first I think.
BTW, it sucks that the other thread “original plating” vanished, because there were some helpful comments in there. in particular, someone was talking about the very details of getting the dust boot over the piston - I hate to ask them to go through it again.
I suppose there is a way I could have deleted it by accident, yeah?
I think they are having some computer problems. Foxpro hasn’t worked for a week or so but IE does and strange stuff happening like getting overloaded often.
Foxpro hasn't worked for a week or so but IE does
I hope you mean FireFox…
Foxpro is a DB.
thanks for a number of your comments, I’ll have to focus on these though by @texases :
@texases wrote:
The pistons must be pristine.
I can now confirm my pistons are pristine. however, I discovered small, localized, but notable pits in the front bores.
I’d like to understand what this pit means - couldn’t the pit serve as a site for brake fluid to boil, as there are surface imperfections that can seed boiling? couldn’t the pit generate fluid turbulence?
@texases wrote:
The bores aren't the critical part, because the seal is between the o-ring in the caliper and the smooth sides of the piston. Is that your caliper's design?
yes that is the design.
now I don’t quite understand the relative importance of the condition of the pistons to the bores. I take your meaning to be that the rubber seal - which resides in the bore - makes small pitting inside the bore negligible - provided the pistons are pristine.
I suppose the critical design here is that the fluid will push against the piston wall and seal. If a defect exists on the piston surface, with a perfect bore seal, the fluid can leak out.
doesn’t it matter where the piston defect is? if there’s a defect on the piston that extends well beyond the bore seal, beyond the area that slides past the seal, I naively think that should not matter, however I can imagine there is some error with where exactly this area is.
again, my pistons are in fact pristine.
I agree with your last statement, Junior…but you said not to tell you that!!!
LOL 
Pits in the caliper bore means new calipers are needed unless the bores are resleeved and the latter is not cost effective unless it’s a rare and expensive, hard to find caliper.
@ok4450 wrote:
Pits in the caliper bore means new calipers are needed…
well that’s grim - but @texases and I think @keith wrote that pits in the caliper bore are not critical … so I am confused now. Are you saying this to “be safe”? can you specify a reason that bore pits are bad?
My thought - if the O-ring in in the caliper, it rubs against the piston, which much therefore be smooth. Minor pits in the caliper bore (away from the o-ring groove) wouldn’t affect the seal between the caliper, o-ring, and the piston. If the o-ring is on the piston, then it’s the caliper bore that needs to be smooth.
People have different methodogies so opinions vary. Mine is based on what I was told at various service schools.
It might be noted that pitting is not always confined to the part of the bore away from a piston seal. Sometimes light pitting exists in the seal groove itself and while fluid may not leak past a piston to seal fit, it may leak between the seal and the edges of the groove.
Consider the cut and paste step in the process below in regards to caliper overhaul.
Odds are with a careful cleaning and reassembly you would be ok. There’s a difference between DIY and doing it for hire; especially with brakes involved.
yes @texases and @ok4450 I can understand those comments.
more thoughts about pits - is the pressure inside the system being applied significantly to the sides of the piston? I don’t know what equations to use to show this, but I would expect the pressure at a pit at least to be uneven against the piston if that is true - that might lead to a tilting piston. I woudn’t think a pit in a volume of brake fluid that is pressing against the bottom of the piston to affect the distribution much.
I can see that pressure in a pit could lead the pit to erode at the edges and become worse, probably quickly.
I don’t thing there’s much of a pressure issue, except when a pit is in the area of the piston/o ring/caliper sealing area. Pits result from water-instigated corrosion, not pressure.
Saving money on the Porsche is a good idea. New calipers are probably expensive.
There is no critical area on an iron caliper. There could be on an aluminum one.
For iron: If you assemble it and it doesn’t leak, you succeeded. If it does leak, you get a new one.
Fine sandpaper will clean up the bore of an iron caliper. You don’t have to bother measuring the bore. The pistons could be a different story but you replace metal ones if the plating is gone.
As I mentioned; if it works and doesn’t leak, it meets all reasonable requirements.
@pleasedodgevan2 they are in fact iron, or something heavy like that. and yes new ones are priced ridiculously.
assembly instructions call for either brake fluid or special silicone lubricant, and I have both. Can I use just the silicone lube? reason being this will take a while, and the fluid will sit around. even though the system will be flushed out, I’d rather not have junk in there to begin with.
also what consequence is there with mixing brake fluid and assembly lube, especially putting it in the system? I need DOT4 but silicone is not DOT4.
They will mix without problems. Silicone is probably the most neutral stuff in the world for this. Read the label and if it says nothing about compatibility, you’re in.
@pleasedodgevan2
Is silicone brake fluid compatible with DOT4 fluids?
If so, that’s a new one on me.
No. You can’t mix the two types of fluid.
hey all - yes I know the brake fluid DOT4 spec is not allowed to mix with the silicone spec fluids - my question was about mixing the assembly paste and the brake fluid. I asked because various instructions call for both at times, without any explanation.
HOWEVER, I sat down and worked on this today and I conclude the assembly paste is sufficient - brake fluid is not necessary, because the official shop manual calls only for assembly paste.
thanks anyways - … I think @pleasedodgevan2 knew exactly what I was talking about, but I realized it would be easily confused… sorry for that.
When a type of fluid is called for, nobody should mix with another. For the record.
Is this thread part of a thesis on obsessive compulsive disorder ?
Another win for drum brakes, The wheel cylinders on old beetles used to be able to be rebuilt by stoned hippies and they never had any trouble, There was never a discussion.
It seems like disc brakes are like a trophy wife, Nice to look at, and a lot of fun when times are good. But they are a good time charlie, as soon as you hit the first bump in the road, the drama starts.
Disc brakes are too much drama for me. Heck, 7 or 8 wheel cylinder rebuild kits used to cover 85% of the cars on the road back in the day.
Brake fluid? All old wheel cylinders required was wet fluid.
People drive like maniacs now and it because their disc brakes make them cocky and arrogant and they lose track of how fast the are going. With drums you always built in a cushion when stopping.