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I realise that this is probably a newbie question...so please go gentle..

When operating the twin piston Tokico calipers are both pistons supposed to move together in a linear fashion..

Or is the caliper designed to allow one to operate until it meets resistance at which point the hydraulic pressure acts upon the second piston.

I'm asking due the way my OSF caliper is currently operating.

With the caliper removed from the hub and hung up - pressing the brake pedal causes the upper most piston to operate (move out) but the lower one remains static.
If I then chock the top piston with a piece of wood and operate the break pedal again - the the lower piston does start to move.

My thought are that they should both move in sync.

The OSF front wheel/caliper/disc are getting pretty hot after a short journey so checked the caliper for movement and found that the top 'slider' was really stiff so replaced it and drove the car again, but same thing happened. It now looks like the disc(s) and pads need to be replaced as the OSF disc has a blue tinge to it from the excessive heat.

It appears as though the lower piston is partially seized and a caliper rebuild is required.

Could one of you help explain the caliper operation and whether my diagnosis is correct please.

Cheers

 

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Yes all pistons should be able to move freely in their bores, but inevitably even with brand new pistons one piston will be slightly freer in its bore and move more easily, gentle hand pressure via a screwdriver holding the moving is all that should be needed to make the other piston move instead.

Opposed twin and 4 piston calipers are the best out there for 'exercising' the pistons in their bores during annual brake servicing, ie take pads out and as you push one piston in the other moves out.

I exercise the 4 piston calipers on my Landcruiser like this, all 4 more or less the same pressure required to move.

Sounds like a rebuild is on the cards.

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Thanks for the reply @Juddian

I'll get some parts ordered.

Just to confirm, the Subaru Forester caliper pistons are 43mm in diameter with the outer dust seal collar diameter being the same as the piston body?

E.G. The outer dust seals do not have a retaining spring

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From what I can tell - the front caliper pistons on my FXT are 43mm diameter, 42mm collar size with a height of around 48mm

48mm height x 43mm diameter piston x 42mm collar diameter

Does that seem correct?

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Sorry can't help you with the sizes, personally if calipers need rebuilding i'd rather let Big Red do the job and paint them at the same time.

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