I add this in the case is useful for someone, I've done a disaster as I put the caliper in a bin with citric acid for removing the rust. If fact, the citric acid works very well removing rust without effort but the problem came from the piston housing that is a kind of alloy and reacted with the acid becaming soft. I thought it was normal steel.
I only cleaned with a toothbrush very softly and I thought I was removing dirty or rust but in fact what it was happening was the alloy melting.
you can see on the picture where it was sharp edge now there are blunt edges.
This part is only there to guide the piston. As the seal is made between the o-ring and the piston, I would not worry too much. You should try to assemble the unit and test the sliding. It should be firm without excess. You must also check that the steel collar can still hold the outer seal. Acid can be used here, but you have to coat the machined surfaces with grease before to protect them.
But I put the parts inside a can for hours. Impossible to mask the parts.
I will follow your advice and give a try, it seems it keeps tight the rubber ring and the outer metalic ring. The inner seal comes from frenkit and has two lips instead of original with only one.
I`m waiting for the castrol rubber grease for assembly.
Topic 
Front brakes stucked. how to repair?
Posted - 11 Jan 2022 : 12:31:59 
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