Hello Baz,
I just changed the front brake piston seals at the weekend.
While it is still fresh in my mind, here are some tips that might help.
I know you have already drained the fluid, so 1st option is to try to blow out the pistons using compressed air at your local garage. If you had a helper I would say use them as I found trying to juggle holding the air hose & hold the nozzle tight onto the fluid intake on the caliper a bit difficult.
1st remove the pads and insert a piece of wood in front of the pistons that will allow the piston to come most of the way out. Now try to get a good press between the air nozzle and the fluid intake and send some air in. If you are lucky, both pistons will come out as far as the piece of wood.
I didn't actually use a piece of wood, instead I use the compressed air to blow out one piston, then re-inserted the piston to just sit into the seal(piston seal, not the dust seal) and held it there with a small clamp. This allows the compressed air to work on the other piston. I would guess a piece of wood might work better if the thickness was just right to allow the pistons to come out as far as possible with losing the seal. Then in order to get the pistons out the last little bit, I used a screwdriver with a handle that fitted snugly into the inside of the piston and a bit of wiggle & pull got them out.
The screwdriver handle was also useful to help refit the pistons - you need to be able to push fairly hard & straight to get them to pop back into the cylinders.
Above was fine until 1 piston refused to budge using compressed air. The only way I could think to sort it was to complete the seals on the caliper where I had got both pistons out, re-fit pads, etc & re-fit the whole caliper.
Then I had re-fit the other caliper to the brake line, re-fit the piston that had come out & clamp it in place. Then I had to refill with fluid & bleed in order to get to the point where I could pump the brakes until all the hydraulic forces were working against the stuck piston - this worked ok but it was a pain to have to refit almost all components, fill & bleed.
In hindsight, what I think would have been best would be to:
* Don't disconnect the brake lines - just loosen & quickly re-tighten enough to get a decent seal.(This allows you to loosen off the brake line at the end as the caliper will be dangling loose and not so easy to loosen a tight nut)
* Remove both calipers
* Remove both sets of pads
* Insert a piece of wood in each caliper to allow pistons enough space to come out without losing the seal(if this happens, the fluid will rapidly drain and won't be able to exert pressure on the other pistons.
* Remove wood pieces when all 4 pistons have extended out.
* Disconnect from brake hose.
* Insert screwdriver handle(that is a snug fit) into inside of piston and wiggle & pull!
To be honest, if you have not already removed the calipers, you might be better off to refill & bleed, and try to get all 4 pistons pushed out as far as possible using the hydraulics.
Good Luck!
George
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