I’m trying to service my old pre-mono Hope M4s before Dyfi. Got the front calliper off and in 2 halves. Got the pistons out of one side using the hose-and-a-bike-pump and thumb-over-the-hole method, but they simply WILL NOT budge on the other side (which explains why they’ve not been working too well recently).

Any suggestions how I can free up the stuck pistons? ANy other way to get them out other than blowing them?

Oh yes, don’t mind trashing the seals as I’ve got the replacement set to go in…

It might be a bit late now but I pump them out with the hydraulics all intact. Ie pads out pump the lever till one moves halfway out – hold that in and contimue pumping until the other is halfway out – them split the caliper and pull them the rest of the way

* I don’t know M4s, but consider the following idea

Firstly check that the piston is square to the caliper – this may be the issue

Could you push it further in – in an attempt to break whatever the ‘seal’ is that is holding it in place – like a gclamp applying gradual pressure, once it moves in (and you could apply lubricant to seals whilst moving them) – it should hopefully move out.

Another idea – as water doesn’t compress as much as air could you try pumping in water ?????

An old motorbike trick is to put things in a freezer they shrink – so theoretically this should loose the piston

bit of dot fluid on the pistons is a good lube

How much air pressure are you using, it took 120psi (from a track pump) to blow out some pistons from a set of Hayes the other day, I had to seal of the small hole with a bit of rubber and a clamp to generate enough force to blow the pistons out. Wrap some rag around the caliper to stop the piston coming out like a bullet..

What I did when faced with a similar problem on an old Mini – when even a compressor wouldn’t pop the piston out ….

Was …

Cut a small screw to just fit inside the piston
I then put a nut on it enabling it to be tightened against the inside walls
I could then use pliers to twist the piston to free it
It was a bit fiddly getting it in & tightened but worked well

Thanks.
Filled it all up and used the fluid to pump the pistons part way out, then managed to get enough air-pressure in it to blow it out.
Seems the lip of one of the cylinders was slightly scored so stopping the piston.

Now I just need to figure out why the hose-connector isn’t sealing to the caliper properly. Seems the brass washer just isn’t doing it’s job.
Any suggestions?

Try annealing the copper washer

Heat it up until it’s cherry red & then quench it in water

That worked!!

New seals, new hose and new fluid and the brake feels amazing! Now the rear….