Brake piston sticking

I thought I might just check my brake pads and calipers to see if there is any salt or crap hiding in there and as I was at it I have ordered some new pads although the old ones are only half done. I removed the pads and there is a small touch of salt residue but nothing to worry about however when I squeeze the front lever three out of the four pistons come out but at slightly different times but 1 of them suckers stays put so I thought do I put a wedge between 2 of them and hope the pressure forces the sticking one out but I bottled it thinking too much pressure may pop out a seal (or worse) So a couple of basic answers please (1) Should they all come out together ?....(2) How do I get the sticking one out ? (3) How many millimetres from the flush with Caliper body do I pump out the pistons before the damn things pop out and wreck my night .Thanking you in advance

 

I'm thinking that my pistons are a bit sticky. When its been sat on its paddocks for a couple of days the front wheel seems to stick when revolved but when it's been out for a ride it spins freely again, I think I might take the calipers off and send them to Bigred for reconditioning. Anybody recommend another company that can strip them down and replace the pistons/seals and make them as hood as new?

 

Take the calipers of mike, and clean the pistons using wd40 or suchlike... Did mine tother day..shouldn't stick then.

 

Remove the pads and give everything a good clean first.

(1) Should they all come out together ?.... mine never all came out at the same time when pads were removed and the lever pumped for cleaning the pistons
(2) How do I get the sticking one out ? remove the res cap and carefully push the two opposing free moving pistons back into the caliper, keeping an eye on the res fluid level, wedge in place, then holding the caliper and the other free moving piston with your hand , carefully pump the lever to force the sticky piston out untill you can see the ring of crap around the piston and clean with a toothbrush
(3) How many millimetres from the flush with Caliper body do I pump out the pistons before the damn things pop out and wreck my night ... the piston will safely push out about 12mm

 

I've never known the pistons to all pump out equally. I'm sure the literature would say they should, but after real world use I think it's doubtful that they will. Gave mine a good clean Sunday when I fitted new pads and had to wedge on set and pump the lever in order to get the second set out far enough to give them a good clean. WD40 follwed by soapy water does them wonders!!

 

Thanks boys I will have a doo tomorrow .On a similar note lets say you are on the bike and brake at a set of traffic lights the pistons push out the pads to grip the disc but what sends the pistons back once you release the lever

 

Fookin ell giv, never thought about that before. I assume its the way the master pressurises the system. Imagine all that fluid being pumped into a small area via a one way valve, fluid squeezes out under pressure but has nowhere to go until pressure (lever) is released, then it naturally 'eases' itself back. They dont actually spring back afaik, which is why the pads are usually just about skimming the discs. Er, probably. Or summat like that.

 

Do i get a point for effort?

 

Wrap an old towel around the reservoir Mike just in case of an accident. In answer to your question, that's how hydraulics work. Imagine pulling the trigger of a silicon gun with a bung in the nozzle, if you keep the pressure on and remove the bung the silicon will squirt out. However, if you pull the trigger then release the pressure and take the bung out no silicon will come out........see .......Magic!

 

I bet someone ends up with silicone over their discs now....

 

I read about this once, can't remember the exact science but you can see it in action when you have the calipers off the bike tomorrow with the pads removed and you push the pistons out for cleaning. Took me about 2 hours on Sunday thoroughly cleaning and changing pads! Don't worry too much about the pistons over extending either, I tried to force them out of my VFR calipers once to do the seals and they don't half go deeper than you expect.

Oh, and here's an explanation for the hell of it:

You squeeze the lever and move the master cylinder piston. You MC piston is much smaller than your caliper piston so you get increased force and decreased displacement at the caliper. There is a spring on the backside of the MC piston that pulls it back to its neutral position of "brake off" when you release the lever. When the piston returns it pulls the fluid with it which pull the caliper pistons with it as well, the effect of the spring is enhanced in the same way application of force at the lever is. Your brakes "self-adjust" because there are small ports in the MC that get covered/exposed at specific points in the MC pistons travel so as maintain a constant distance between the rotor and pistons by allowing the system to suck in more fluid to make up the wear in the thickness of the pad material. That's why you have a reservoir.

 

Pistons will not move out equal with no load if you place a block in the caliper they will all be under equal force, you should never be able to burst a seal no matter how hard you pull the lever if you block or hold the moving ones and the other still does not move with max force then you defo have a problem when the pistons are all under load they force on each will be equal as the fluid is incompressible thus force on the system is the same throughout, any pistons not under load will or should travel until they are.. The reason they don't move at the same rate is the dust seal have drag and the drag on each is different, hence why they often drag the pads a little when you spin the wheel and why full fat racing calipers have no dust seals, good for less drag but bad for dust...lol

 

Use quality brake cleaner instead of WD40, thats not a great thing for the seals, they are a natural product, and the make up of WD40 is not the kindest thing for them. Also lightly smear red rubber grease on the pistons after cleaning them, and before pushing them back in, it lubes the seals but does not attack them like WD does, they stay more supple so you get less drag on the pistons, hence less rub on the discs, most brake drag if the caliper guides and pins are clean is caused by dirty or swollen dust seals, nothing else.

OOPS did not scroll down far enough see Arthurs explanation above before posting

 

Kentblades bang on with the red grease I use it all the time will refurbs.. you can get handy tins from eBay

18g Castrol Red Rubber Grease Brake Caliper Fluid Handy MINI TIN

 

Nice Arthur, I bought a 500gm tin about 10 years ago, by the time I use all, I calculate I will be about 350 years old

 

Same....lol