Sticking caliper piston, maybe?

[daverclasper]

Hi. Recently noticed that car is pulling to the left when braking.  New rear cylinders a few months ago and were copper greased and sliding freely, so thought I would investigate the front offside caliper.  So with the caliper tied out the way and the pads removed, the inner piston, close to the brake pipe is moving in using the pedal, though the outer stays put, though it will move in if the inner is clamped.  Have used penetrating fluid and moved it in and out a few times, though not made a difference.

I guess the outer pad will start to work as soon as the inner pad is firm against the disc in operation, though enough?.

So just wondering if this is normal and if not, what I need to do.

Any help great please.

Thanks, Dave   

Edited May 31, 2019 by daverclasper

[NonMember]

I've not tested it but I wouldn't be at all surprised if that behaviour were normal, or at least not unusual. The inner piston sees the pressure first, especially if there's any air in there. Given the design, it's almost impossible to replace the last bit of fluid in the outer piston short of a full rebuild, so it's marginal as to whether it can ever be properly bled.

Instead of clamping the inner piston, try a few hefty rubber bands. If that's enough to make the outer one move then all is probably fine. If the outer is stiff enough that the inner can stretch the rubber bands instead then you might need to improve it.

[Pete Lewis]

when on the disc the pressuer i about equal on both as one reacts against the other , out in fresh air then i reckon you get what you are seeing 

the easiest moves first . Nm idea of rubber bands is worth a try , but you may need a few ,

as you say when   clamped it moves  .  remember there is upto 1500 +  psi in a brake system when pressed hard 

an slightly stiff piston is no contest over the load you apply when braking 

try italian tune up   60-20mph  hard stops repeat 3 times , you may just have rust embedded in the pad

pete

[GT6M]

If yer cars pulling t,ya side, IE  pull t,left,

then its no that  side caliper at fault,

its t,other side caliper

 

I see yer saying that its off side { right , drivers } took t,bits though, correct

 

but give yer pads an discs a good check,

rusted discs, ridged disc,ridged pad, bearing play,

 

M

[daverclasper]

Thanks everybody.  Well,  clamping the free moving piston with my thumbs and can't stop it moving out and tother one not moving.

Going on what Pete saint about the hydraulic pressure, then is it worth popping it out and further cleaning do you think. If so I assume it is easy to refit?.

Cheers, Dave 

[Colin Lindsay]

You can pop one out, but the other is always much more difficult as you need to block the hydraulic or air pressure to force it out. If, as I've done many times, you use your hand to block the pressure in the empty side, watch your knuckles as the other piston can quite painfully pop out and strike your hand. Both sides are easy to clean, but much easier to get at if you split the caliper once both pistons are removed and do the two halves separately. It's also a lot easier to replace seals etc this way. If you do split them, look out for the small rubber o-ring seal that seals the hydraulics between the two halves, replace it too as it only costs £1.50 approx from suppliers - Canleys refer to it as part CS1 - middle of the lower three holes in this photo.

 

[DanMi]

If you are going to go to the effort of taking the piston out, I would put a new seal kit on. If it is sticking there is probably corrosion on the pistons so I would bank on replacing those too. I have just levered the pistons out with 2 screwdrivers before but that does knacker the lip of the piston but as I said If I'm going this far they would be scrap anyway. 

[Colin Lindsay]

If they were that easy to get out, that's not bad at all. THIS one had to be drilled for a metal bridge then chiselled out without causing damage to the caliper, and was certainly scrap afterwards!

[Jezza]

I had a similar problem with my modern car (E46 M3) and the garage I took it to said that the piston seal had failed leading to grit and grime getting in between the piston and the caliper housing (where the piston moves back and forth). A common problem on these cars apparently.

It causes the outer side of the piston to become scared or marred to the point that it retains the grit and grime and eventually seizes. The only solution was a complete rebuild of the caliper (and likely continual rebuilds from time to time to clear the grime out) or a replacement. In addition, as this seizure had been happening for a couple of months the heat build up caused by constantly having the brake applied meant the disk was beginning to warp and the pads were starting to degrade due to excessive heat (the disk was reading over 250 Celsius after a 30 min drive compared with 37 Celsius for the other side). An eye watering repair cost (for the car it is) for a caliper, 2 disks and pads, but a quick one to do.

[daverclasper]

Thanks. It seems it might be a rear brake that's not working. Have created a separate thread, as it isn't obvious why.