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MG MGB Technical - Brake Caliper Pistons - Removal

Anyone have any tips on removing particularly stubborn pistons from a disk brake caliper? With the unit still on the vehicle I used the hydraulics to get them about half way out, but didn't go as far as pushing one of them all the way out. I didn't feel like dealing with the mess and besides, I'd still have to deal with the other one.

Curt

cjd DeHaven

It depends on whether you want to separate the two halves of the caliper. I prefer to do it that way.

I use a foot-pump, although you could use an airline on low pressure. I use a piece of an old inner tube with a hole in the middle to act as a seal. You will need some rag to catch the piston and any fluid that is remaining.

If the pistons are already moving, it won't take much to push them out.

If you don't want to split the caliper, you will need to remove and replace them one at a time. use some strong wire to retain the piston that you aren't removing and once you have refitted one, swap the wire over and remove the other.
Dave O'Neill 2

The next step is to remove the caliper, cover the brake line with a short rubber hose with a bolt in the other end to keep the brake fluid in the system. Hold the piston that you can get out in place with a C clamp. Install a grease fitting in the fluid inlet hole and use a grease gun to force the stubborn piston out of its bore. RAY
rjm RAY

Wow... Greese gun - interesting idea... 'Cept you end up getting greese into the caliper, no? I was always taught to keep petroleum products away from places where brake fluid lives...

I did split the caliper. Getting the outer piston out was straight forward. I ended up using a modified approach of Dave's. I used my airline with an old valve stem - from a tire/wheel. I was able to hold the rounded end against the cylinder's inlet easily forming a good seal, then applying air from an airline on the other end.

The inner side was a bit tricky cuz of the 2nd hole that goes over to the other half. I ended up plugging that with a piece of flat rubber and then bolting the two halves back together - to hold that flat piece of rubber in place.

Question: I'm ordering a repair kit. So how do u get the seals out of the cylinder - just what all comes out? More importantly, how do u get the new seals in?

This is all happening because when replacing the pads it was real tough moving the pistons in for the new pads. In fact I couldn't get the outer piston all the way in... And one of the pads had excessive wear suggesting that the piston wasn't backing off when the brake was released? Now that I have everything apart, the pistons look to be in really good shape - very clean, very shiny. The cylinders don't seem to be too bad, something I can work with and clean up. Though, there is a bit of solid junk in there... Comments welcome.

Many thanks for the help!

Curt

cjd DeHaven

That "solid junk" was preventing the pistons from returning all the way back in to the bore. You can gently pry out the old bore seals with a dull pick or small screwdriver. Make sure that you clean the groove, that the seal resides in, as best as you can or the the piston will bind up and cause the brakes to drag. RAY
rjm RAY

YES! Ray, great observation. I think you're right about the "solid junk". When I went back to the bore that refused to let the piston seat all the way in, the solid junk really was solid - it wasn't just sludge. I had to scrape it out... I pulled the seals out per you guidance. I noticed the ridge between the seals had some "stubble", but a piece of emery paper made history of that. Some other general clean-up and lubrication and the pistons now move very nicely in and out. Sweet. So now I await new seals...

Many thanks for the help.

-- Curt


cjd DeHaven

I always clamp the inner piston & use compressed air (grease gun, tyre pump)to remove the outer piston, split the caliper,use a 7/16 short bolt & large washer to cover the transfer hole & then blow out the inner piston. I use a piece of 1/8 wire bent with a 1" leg on one end & with a small flat ground on the bent end that will fit in the outer part of the groove to scrape it out.It is the outer face of the groove that you can't see without a small mirror that gets all the dirt stuck on it. Use rubber lube on the piston & seals before assy. Very often the outer seal retaining ring has a burr on the edge that makes it difficult to push home without bending so I always run the corner of it over the grinding wheel - a file will do the job.
Garth
Garth Bagnall

I put stainless pistons in mine about 10 years ago, from the MGOC, no problems since. An airline and a clamp gets them out fine without splitting the caliper.
Stan Best

This thread was discussed between 05/07/2010 and 09/07/2010

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