All brakes are engaged

[jbcollier]

To be honest, I don't rebuild brake master cylinders any more.  I just replace them.  If the bore was in very good condition, the rebuild went well.  That was fine for five or even ten year old vehicles but with 40 year old vehicles, the bores are almost always worn too much, and it's not always easy to detect the wear.

Other manufacturer's rebuilds?  Forget that, right out of the box half didn't work or failed again within the week. 

[CCM911]

Be sure that your Master cylinder has the removable reservoir.  Mine was not removable.  Believe it or not, the instructions that came with the rebuild kit were pretty good.

Use HOT water when removing the m/c resevior as it is easily cracked.

[Grumblebuns]

I believe both stock MCs for the S2s and TCs were somewhat removable. The front nipple of the reservoirs popped into the front port that had a groove with a wire clip which the nipple "popped" into to secure the reservoir. To remove the reservoir, you had to heat up the front plastic nipple (which involved soaking the master cylinder in hot water to soften the nipple) then carefully pry the nipple out of the port with a couple of screwdrivers. The other method is to drill a small hole in the side of the front port to split the metal wire clip.

The attached picture shows a S2 MC reservoir with rubber seal but not the wire clip. You can see the groove in the front nipple where the nipple "pops" into the wire clip. If the plastic is softened sufficiently, I believe you can remove the reservoir with minimal damage.   

[bozman]

UPDATE:

Pulled the front calipers off last night.  The pistons were seized into the caliper.  I was able to break them loose, then use the air compressor trick to get them out on the left front caliper.  The right front was a little more challenging, so that has been soaking overnight and I'll give it a go tonight after work.

[E Paul]

I had to pry mine out when this happened to me. 2 of the dust seals had failed enough to allow water to get in and rust the pistons outside of the square o-rings. When I changed brake pads last year and compressed the pistons, that left me with dragging brakes which caused lots of smoke! 140 PSI was not enough to push out the stuck pistons !!! I rebuilt with stainless steel pistons from RD. All good now.

[4129R]

What happens when you push the pistons out with hydraulic fluid? Surely that is better as fluid, unlike a gas, is virtually non-compressible.

Just connect the caliper to the brake master cylinder, and push the pedal.

Alex in Norfolk

[RoddyMac]

At the shop I used to work at, we had a pedal box with master cylinder and a flex hose to attach to calipers to aid in removing pistons.  You would simply pump up the master cylinder (via the pedal attached) and push the pistons out.  If one moved and the other didn't, you applied a C-clamp to the piston that moved to put more pressure on the stuck one.   If that didn't work, then we just tossed the caliper and found another one in the caliper core bin.  Though that typically didn't happen often.

[Grumblebuns]

I tried using a brake piston removal tool to remove a stuck piston. The handles were way to short and flimsy to get adequate leverage to grip the piston hard enough to remove it. Finally gave up and reattached the caliper to the brake system and used the MC to press out the last piston. Obviously I had to reinsert the piston on the other side and retain with a "C" clamp. I'm still looking for an effective piston removal tool.   

[4129R]

You two have confirmed what I was going to do. Thank you.

[bozman]

In my case, I used a portable plug-in air compressor that I purchased at the hardware store ($60 from Slime) for filling tires that came with a cone shaped end adapter. After the calipers were completely removed and on my bench, I broke the piston free in my vice (pushing them further in) then sprayed them with penetrating oil. I put a heavy screw driver in the brake disc slot to keep the first piston from popping out before the other one. I then placed the cone adapter end into the fitting on the side of the caliper and after about 120 psi they started to slide right out.  I was actually pretty amazed at how effortless it was.

I have seen the fluid method too, but this requires no messy clean up.

[E Paul]

I used a grease gun.

[4129R]

I used a grease gun.

I could not get the end of the grease gun big enough to put over the caliper bleed nipple.

I used a brake master cylinder. Very messy and took ages.

Alex in Norfolk.

[bozman]

Just use compressed air.

No mess, no worries. 

[4129R]

Just use compressed air.

No mess, no worries.

Compressed air didn't move any of the 4 pistons.

[bozman]

Just use compressed air.

No mess, no worries.

Compressed air didn't move any of the 4 pistons.

In may case, I put the calipers in a bench vise and I was able to break the pistons free by compressing them back into the calipers more.  Once that was done, I hooked up the compressor.  The PSI got up to around 130 psi before they started to move.