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Guest Message by DevFuse

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Blowing out rear caliper pistons without compressed air.

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2 replies to this topic

#1 Corvid


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Posted 04 July 2013 - 05:01 AM

I was helping a buddy take down some rear calipers he got off a GL-10 tonight, when we realized that the pistons were stuck in place with corrosion and/or filth. Bummer. After some cursing and troubleshooting, we ended up pulling out as much of the top seal as we could, hosing down the piston with break-free, loading the caliper into a 14-inch C-clamp, and then compressing the piston to the bottom to break it free and deliver solvent to the cylinder.


Ok, so now what? We have crunchy, loosened pistons that still won't come out, and we don't have an air compressor. Well, we do have all my stuff for working on road bicycles...


We put the rubber brake lines back on the calipers, and removed the left over 10mm fitting from where the body-mounted hard brake line got severed. After cleaning up the cut end with a dremel, we got the small piece of flared metal tubing out of the 10mm fitting, and dropped the fitting in a small vice. In about 10 seconds, we drilled the hole through the fitting out to almost 1/4", so that it would barely fit a Presta valve from a road bike inner tube. We cut the Presta valve out, so that there was just a tiny rubber donut at the bottom, which sealed against the flared opening on the brake line leading into the caliper. We snugged everything down, threw the two-handed floor pump on that bicycle inner tube valve, and cranked it up to 130psi. A little prodding, and both stuck pistons shot out like cannonballs.


Now we just need to hone the cylinders, order new pistons, reseal the little demons, and its rear caliper conversion time. :horse:


On another interesting note, these calipers have the less common 30mm pistons, even though they came off a 4wd GL10.


I'm not sure there is any rhyme or reason that I understand behind the difference.

#2 ruparts


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Posted 04 July 2013 - 12:19 PM


 i don't know how many different diameter pistons they made in the rear disc systems for the 80's & early 90's, loyals ,gl,  gl-10, xt, xt6, series  of subarus,  i know i personally have encountered 3 different sizes of rear pistons.   i suspect there might be a couple more ,  basically i guess it is not an issue which ones you use just so you put the same size on both sides of the car.   technically the systems were tuned with the proportioning valve , but for most of us the difference would not likely be noticed,  imho. 

#3 Corvid


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Posted 04 July 2013 - 12:40 PM

Yeah, that seems to be the case. The main concern I've noticed is finding the rubber and pistons for rebuilding, like I listed in that link up top.

For that reason, I feel that the sets with 34mm pistons are the most desirable.

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