W123 caliper seal change

[stuarth]

I dismounted the calipers since they are jamming and overheating the brake. I have removed the pistons (on one of them) and now need to change the seals (I assume) and the rubber boot on each. One of the pistons slides in and out ok but the other does not and is virtually impossible to push down even though it seems nice and shiny and smooth. Any ideas why this could be or the remedy?
Any hints on doing this job? It looks pretty simple at first glance.

thanks

 

[television]

So you have not got one piston out yet

 

[stuarth]

One of the pistons moves smoothly so I did not remove it. The other was jammed in so I slowly levered it out. I have polished with 1200 grain emery paper where needed. I removed the rubber sealing ring and the piston now goes in and out fairly well so I assume it is or was the rubber sealing ring (even though it looks brand new).
Now I have come this far I guess the seals all need changing.
I have not split the caliper - don't see why given that removing the pistons doesn't require this.

 

[television]

Yes, never refit the same seals and never assemble dry as the seals will be destroyed,many use brake fluid but silicon or PTFE is much better to use these days and is good for rubber

 

[wireman]

Do not use silicon fluid in ABS equiped cars (unless the makers specify it) the fluid's elasticity can make the ABS get confused whilst its doing its best to control the fluid pressure. The elasticity lowers the resonant frequency of the hydraulic system to one which could be less than the pulse frequency of an active ABS unit forming a low pass filter between the power source and load and therfore reducing the efficacy of the ABS units rapid hydraulic pulses, this condition will vary according to pipe length and be different at each wheel, this is not an ideal situation to have under emergency braking.

Do not split the caliper it is not required and you will not be able to be certain sure that the seal between the caliper halves is good, the manufacturer prohibits this for good reason.

Wash brake parts with methylated spirits and lubricate for assembly with brake fluid only.

If you are fitting new seals you realy should also be fitting new pistons.

If the emery polishing bit refers to the old piston throw it away, the chrome plating will have become scratched (even a scratch too small for the Mk1 eyeball to discern is to big) and its long term reliability severely impaired. Emery and silicon carbide abrasives can leave small traces embedded into the surface against which they have been applied resulting in progressive degradation of the so called cleaned up surfaces whilst in service, the parts must be cleaned of this material before any attempt is made to reassemble the parts.

If you can get them I would prefer to see new or manufacturer rebuilt calipers used for this repair.

When all is said and done there is no benifit to society in having cars with dodgy braking systems running around, despite brake failiure being causal in a very small proportion of incidents and then not always with any devastating results.

The service brake must work correctly when called upon to do so, fitting questionable parts can only reduce reliability, driving carefully because you know the brakes ain't what they should be is criminal.