Educate me about brake caliper piston machining (SS vs Alu)

[Holescreek]

Interesting! I knew about seal distortion (with seal in bore) causing the retraction of the piston, but I wasn't aware of the angle at the bottom of the seal groove. Is that a common feature in all SiB calipers? And presumably the angle is oriented to make the seal tighter at the front (towards the disk) side? Or is it the other way?

And for that matter, how do the seal on piston calipers retract? If anything, deformation of the seal should make things worse, right?

Did your company make complete caliper assemblies, or just the pistons?

The seal groove is tapered shallow at the front so when the pressure is released the rubber seal relaxes backwards pulling the piston back. The piston doesn't need to move a great distance in retraction, just enough to take the pressure off of the brake pads. The test for this is called drag torque. The customer specs determine the maximum amount of residual drag torque allowed after cycling the caliper at a specified pressure.
The company makes complete OEM disc brake calipers, electronic parking brake calipers and ESC (electronic stability control) units.

To the OP: Sanding pistons is a really bad idea in my opinion. I understand that you did what you felt was necessary at the moment. Piston diameter and surface finish is critical to caliper function. The caliper seal only rides in a small area of the piston but this area changes over time as the pads wear. You might be in a "good area" of the piston right now (but maybe not) but when the pads wear another 5-6 mm the seal will be riding that much lower on the piston too. Fluid seepage past the caliper seal can build up until it fills the piston boot before you even start to notice it on the casting so keep an eye on the fluid level in your brake reservoir.

 

[dian]

i thought the notion of seals retracting the pistons was a misconception/whishfull thinking. you seem to know what you are talking about, though. but i still wonder: specs for disc runnout are up to 3 thou. all it takes is one thou to free the pads. what do you make of it?

btw, i have been polishing up pistons for decades, some of the cars are still in the garage. no problems whatsoever. (of course you cant "sand" them down to another dimension.)

edit: the spring action of the rubber is unpredictable. how much will it retract? if too much you get a spongy pedal. no?

 

[Holescreek]

i thought the notion of seals retracting the pistons was a misconception/whishfull thinking. you seem to know what you are talking about, though. but i still wonder: specs for disc runnout are up to 3 thou. all it takes is one thou to free the pads. what do you make of it?

btw, i have been polishing up pistons for decades, some of the cars are still in the garage. no problems whatsoever. (of course you cant "sand" them down to another dimension.)

edit: the spring action of the rubber is unpredictable. how much will it retract? if too much you get a spongy pedal. no?

The seal groove in the piston bore has a "Safety Critical" designation. If you were to cut through the groove and look at it from the side you'd see the two parallel sides I mentioned, and the tapered bottom in the groove. each of the parallel sides also has a chamfer in the bottom corners and at the top. The top chamfer at the rear of the groove (towards the bottom of the bore) performs no mechanical function but the chamfer in the front of its space when the piston is extended so it will flex back when pressure is released. Because the groove is Safety Critical the QC lab has to contrace (we use Mitutoyo CNC contracers)the groove produced from every lathe once per shift and after every tool change. It's quite laborious just having to load and unload parts from the contracer when you have 2 lathes on each of 24 machine lines.

As to far as how far the piston rolls backwards when pressure is removed? I don't remember ever seeing a spec on the physical movement since we are only required to test drag torque on completed calipers. Having the piston roll back too far as you mentioned, should have no effect on the pedal since the hydraulic circuit remains closed. The spring back of the rubber seal is more consistent than you might think, drag torque issues I've dealt with in the past were never related to the rubber seal, almost always the shape of the seal groove. But I was typically dealing with newly assembled calipers in the factory.

 

[Bill D]

It only need to retract less then 0.005" or so. I think that is a good figure for rotor runout. The rotor runout does most of the work so it only hits at one point then the rubber pulls it back just enough so nothing is touching.
Bill D

 

[dian]

" The rotor runout does most of the work": thats how i see it.

"Having the piston roll back too far as you mentioned, should have no effect on the pedal since the hydraulic circuit remains closed":

here im afraid you are mistaken. if the piston has to take up any slack the pedal goes to the floor. would be typical for a wheel bearing situation. i even had to install a check valve once that would hold a certain pressure on the pads, was 15 or 30 psi i believe, was a racing application.

thanks for the explanation concerning the groove, you make me curious, i have to check it out, but only have some "offroad" calipers to look at right now.

 

[SPG-01]

Hi Guy's, just stumbled across this post. I design brake systems for a living and reading this thread is a little scary.
Basics - OEM pistons can be made from Phenolic resin, forged steel (plated), stainless, aluminium, titanium, depending on application and target costs.
Surface finish, cylindricity and size is critical for pistons and seal material, shore hardness, and cross section is critical for the hydraulics to function correctly. Get it wrong and brakes can fail, by overheating or loss of pressure.

The typical single or two piston sliding caliper for an OEM application (yours) costs about $30 to manufacture and probably sells for $100/$120 retail.
Please, put those old calipers in the trash and put a new set on. It's not worth tinkering with a safety critical item like this.
A relevant example - light pick up trucks (service vehicles) used in mines, trash and replace the calipers every brake change. For extreme conditions of course. Its less expensive than servicing.

FYI - I'm testing a new piston and seal design now. I'll run 200,000 cycles on the test rig before it goes on a vehicle for further testing.

 

[Bill D]

Hi Guy's, just stumbled across this post. I design brake systems for a living and reading this thread is a little scary.
Basics - OEM pistons can be made from Phenolic resin, forged steel (plated), stainless, aluminium, titanium, depending on application and target costs.
Surface finish, cylindricity and size is critical for pistons and seal material, shore hardness, and cross section is critical for the hydraulics to function correctly. Get it wrong and brakes can fail, by overheating or loss of pressure.

The typical single or two piston sliding caliper for an OEM application (yours) costs about $30 to manufacture and probably sells for $100/$120 retail.
Please, put those old calipers in the trash and put a new set on. It's not worth tinkering with a safety critical item like this.
A relevant example - light pick up trucks (service vehicles) used in mines, trash and replace the calipers every brake change. For extreme conditions of course. Its less expensive than servicing.

FYI - I'm testing a new piston and seal design now. I'll run 200,000 cycles on the test rig before it goes on a vehicle for further testing.

I agree with you in theory but the op has no mail service in his country. He has to make it work until he can go to a different country that may have parts in stores.
Bill D

 

[SPG-01]

OK, go for nominal size less 0.1mm and use 304 or 316 stainless. Can get away with 4140 steel for a short time (corrosion). Make sure the inner corners have a decent radius to allow fluid to pass when fully retracted (R1 to R2). If possible polish the outer diameter to Ra 0.8 for seal adhesion and reduce wear.

 

[dian]

OK, go for nominal size less 0.1mm and use 304 or 316 stainless. Can get away with 4140 steel for a short time (corrosion). Make sure the inner corners have a decent radius to allow fluid to pass when fully retracted (R1 to R2). If possible polish the outer diameter to Ra 0.8 for seal adhesion and reduce wear.

spg, what do you say to the disc having no runout and the piston relying on the seals to pull it back?

 

[Laverda]

Hi Guy's, just stumbled across this post. I design brake systems for a living and reading this thread is a little scary.
Basics - OEM pistons can be made from Phenolic resin, forged steel (plated), stainless, aluminium, titanium, depending on application and target costs.
Surface finish, cylindricity and size is critical for pistons and seal material, shore hardness, and cross section is critical for the hydraulics to function correctly. Get it wrong and brakes can fail, by overheating or loss of pressure.

The typical single or two piston sliding caliper for an OEM application (yours) costs about $30 to manufacture and probably sells for $100/$120 retail.
Please, put those old calipers in the trash and put a new set on. It's not worth tinkering with a safety critical item like this.
A relevant example - light pick up trucks (service vehicles) used in mines, trash and replace the calipers every brake change. For extreme conditions of course. Its less expensive than servicing.

FYI - I'm testing a new piston and seal design now. I'll run 200,000 cycles on the test rig before it goes on a vehicle for further testing.

Making brake parts is not scary when you have no other option. I don't think you realize that for many vehicles made not that many years go, new brake parts are not available. And making new parts yourself is the only option. Many things made more than 25 years ago that have hydraulic brakes, you can't get any parts at all unless they made millions of the car. If it's something they only made a few thousand of, your screwed.

I am currently working on a Italian motorcycles made in 1988 with Brembo brakes of which not many were made.

The front brake master cylinder is not available but rebuild kits are but for how much longer?

Rear brake master cylinder, nothing is available. I had to make a new piston and sleeve cylinder with stainless. I was able to find seals of the correct size.

Front brake calipers are not available but rebuild kits are. And again how much longer?

Clutch master cylinder is also not available but rebuild kits still are.

Rear brake caliper, nothing is available.

Front and rear brake discs, nothing is available. I had to make a new rear disc when the old one cracked.

So without a machine shop at home to make brake parts, I would have a bikes that can't be ridden!

 

[mmurray70]

I realize this is an old thread but I bet a homemade SS piston would be light years ahead of the junk you get when buying rebuilt calipers from the aftermarket guys.

If anybody here ever needs a caliper buy OEM if anyway possible. Aftermarket seems like a great deal until they seize up 6 months later and now your pads are gone too, rotor is warped, week later your wheel bearing goes because it got cooked with heat. Then you try and change that and realize threads are messed up on CV axle, gotta change that too. Aftermarket caliper isnt such a good deal after all lol.

 

[PackardV8]

he has to go somewhere at times and were ever that is there is amazon they will send it . i use to work for a guy that did euro automotive machining they would take gullwing 300 heads and grind out arias that you could almost stick your fist in to and weld it back up . but why i ask ? his reply . were are you going to get a replacement ? then there were the one that came in with the fix my burned out light bulb mentality . to that he would say NEW WILL FIX IT ! and in this case NEW WILL FIX IT ! after all you buy gas , you have food , shoes . your reading this on your phone so if you can get all that other stuff you can get a two bit new caliper

 

[PackardV8]

Basics - OEM pistons can be made from Phenolic resin, forged steel (plated), stainless, aluminium, titanium, depending on application and target costs.

Our brake engineer above can weigh in with an expert opinion, but from practical experience, phenolic pistons were designed because in humid climes steel, stainless and aluminum pistons all fail in various modes. I seriously doubt the OPs phenolic pistons were absorbing any moisture. I do agree the problems are as likely to be caused by sticking sliders.

And yes, in very humid climes, brake fluid should be flushed after each rainy season.

jack vines, who first learned about hydraulic system problems in humid climes in 1970 in Panama and Viet Nam.

Army Experience with Silicone Brake Fluids 780660

A program to evaluate silicone brake fluids for use in military vehicles operating in various climatic conditions was begun in 1967. After 6 years of laboratory testing and numerous improvements in the characteristics of these fluids, a vehicle field test was initiated in the spring of 1973. Three candidate silicone fluids were selected and installed in M151 1/4-ton cargo vehicles, and M-715 1-1/4-ton vehicles operating at the Tropical Test Center (TTC), Panama Canal Zone, the Yuma Proving Ground (YPG), Arizona, and the Arctic Test Center (ATC), Fort Greeley, Alaska. Brake parts were inspected after 1 year of operation, and all parts were removed and inspected after 2 years of operation. After 2 years of service at TTC and YPG, the two water intolerant silicone fluids were significantly better than the water tolerant silicone fluid and the conventional VV-B-680 fluid. At ATC, the water intolerant silicone fluids were removed from test after 8 months due to possible crystallization at temperatures below -50°F, and two new silicones were installed. The new low temperature silicones were superior to the MIL-H-13910 arctic brake fluid after 1 year of service.

 

[Conrad Hoffman]

I used a silicone fluid in my old Triumph (car) with great success. No more hydraulic clutch or brake problems. But, I understand that you can't switch to a silicone fluid in ABS systems, so be careful with what you use, and that it's compatible with what you've got.

 

[gustafson]

I used a silicone fluid in my old Triumph (car) with great success. No more hydraulic clutch or brake problems. But, I understand that you can't switch to a silicone fluid in ABS systems, so be careful with what you use, and that it's compatible with what you've got.

And you still have to bleed it occasionally
Silicone does not absorb water, but water happens, so you still need to bleed it through once in a while so water does not create a bubble in just the wrong place and cause rust and the seal does not like that and...
ask me how I know...

 

[BT Fabrication]

It only need to retract less then 0.005" or so. I think that is a good figure for rotor runout. The rotor runout does most of the work so it only hits at one point then the rubber pulls it back just enough so nothing is touching.
Bill D

most retract about 0.010" for clearance typically.