caliper pistons

[Fizzy]

been trying to get my friends mkIV spit into a condition that he can drive it to the MOT station, and yesterday we replaced shoes and one of the cylinders on the back, and then moved onto the front to replace the pads and check them over, as the car hasnt been used in a while.

we got the old pads out, but using a G clamp couldnt get the pistons to budge at all back into the caliper. he pumped the brake a few times and the pistons moved out a bit, some more than others, but we still couldnt get them to go back in. tried loosening off the bleed nipple, pistons still wont press back in.

we suspect they are shot, but i just wanted to check. the car has some rust and some bad repairs, and he doesnt want to splash out on new calipers only for the MOT guy to say its not worth saving.

i assume the pistons should move 'freely' back into the caliper under the force of a g clamp, like most brakes ive worked on. ive never actually done mine, so cant compare them.

if they are shot, is there a rebuild kit or something for them ? is it easy enough to do at home without a proper workshop ?

edit - right just seen the pistons and repair kits at canley, the kit looks like it just full of seals and clips, looks quite fiddly and easy to mess up.

[drofgum]

Jason,
The worst thing about fitting the seal kit is usually trying to fit the seals that protect the pistons from road splash. However if the pistons have  rust they need replacement.
                                                            Best of luck,
                                                            Paul

[Fizzy]

right ok, thanks. might ring round the breakers just to find a pair that work well enough, that can be refurbed if the cars a keeper. the pistons are definitely a bit rusty, and the big seal on the outside f the piston is perished.

[mikeyb]

You will repair a couple of calipers for around half the price of a recon pair.   Any old caliper is likely to have some rust on the pistons, the other problem is rust on the body between the dust seal and piston seal.  This contibutes to the seizure as it is part of the bore that the piston moves in.

Overhaul is pretty straightforward, I have done loads now and only been beaten by one particularly manky GT6 pair!  Getting the old pistons out is tricky as you have found. I picked up an old mini brake master cyl from a scrappy, mounted it onto a block of wood with a lever fitted to operate it and an old brake flexi hose.  When filled with old fluid, I use it to push out one piston with the other clamped in place. I wet and dry the rusty piston and offending bore area above the seal as described above then refit the now free moving piston, clamp it and repeat on the other one.  With both pistons free moving it is then easy to pop them both back out.

Thorough cleanliness is key, clean outside of caliper before dis-assembly, make sure all internals cleaned with meths or brake cleaner before fittng new bits. Wet new seals with clean brake fluid before putting new pistons in and most importantly pop a bit of brake grease such as castrol red in the area where the dust shield fits - that should prolong the onset of corrosion on the body there.  Incidently most new/recon calipers are unlikely to have been greased here so popping off the dust covers to apply is worth doing if you go down that route.

Do not attempt to remove bolts that bolt the two halves of the caliper together!

Simples   :)

[Paudman]

Pistons will push back in under finger pressure, providing the bleed nipple is removed! Pistons which require forcing in will also require forcing out again....

[willows40]

Jason

i have a pair of calipers here of a 1500 spit which i think are ok if you want to come over.

cheers andy

[Paul Garvey]

MikeyB - Do not attempt to remove bolts that bolt the two halves of the caliper together!

??

I'm doing mine at the moment, took them into a brake specialist to blow the pistons out for me - they split the calipers as a matter of course - in order to replace the rubber washer sealing fluid that flows between halves.

Paul

[mikeyb]

Paul_Garvey wrote:

MikeyB - Do not attempt to remove bolts that bolt the two halves of the caliper together!

??

By doing this you start to fall outside the scope of a home DIY overhaul to replace rusty pistons!  These seals do not come in the usual overhaul kits and the bolts must be replaced with new - I believe they are special "stretch as the torque" bolts.
Yes I would expect a professional reconditioner to do this - that's what you pay the extra money for   :)

[Tinweevil]

MikeyB wrote:

I believe they are special "stretch as the torque" bolts.

Urban myth I believe.  Before I did my GT6 calipers the markings on the bolt heads were still visible, they were perfectly ordinary items tho i don't recall now which grade.  I was still umming and ahhhing about whether to have a go until I needed a pair in a hurry for my dolomite, they came with exactly the same grade of bolt fitted.  When assembling just search online for generic torque settings, there's plenty of engineering sites that give the numbers based on fixing spec and material.