This month’s article is from the  How-To guide from the December 2014 Soft Top Hardtop, available to club members HERE. It’s a follow up from the technical demonstration at the Club’s National Rally at the Heritage Motor Centre. Those of you that saw the afternoon session will be aware of the problem I experienced retracting the rear brake caliper piston. For those of you who weren’t there, the usual method of winding the piston back into the caliper body using the 4mm hex adjuster didn’t work and I had to assist the piston’s retraction with a pair of water pump pliers. I have stripped down the offending caliper to ascertain what caused the piston to fail to retract.

Remember, everyone, any work you do to your car is entirely your own responsibility. If in doubt, you should check technical advice with an independent, qualified person who has seen your car. The MX-5 Owners Club, its officers and forum contributors accept no responsibility for any damage caused to your person or property as a result of you following or not following the advice offered.


Once I’d removed the caliper from the car, it became apparent from the bleed screw arrangement, the finish of the casting where the brake pipe union mates and the type of piston fitted, that this caliper had been refurbished previously

I was guessing that the piston would be rusted, causing it to be tight in the caliper body and as a result I would find that the threaded insert inside the piston that the adjusting bolt screws into would be separated from the piston. I wound the adjuster clockwise to extend the piston from the caliper body.

My suspicions became more aroused as the piston extended. Visible through the brake pad inspection hole I could see that the piston dust seal had become dislodged from its recess in the caliper body, allowing water ingress and now, visible rust.


Once the piston was fully wound off the adjuster bolt I removed it by gently pulling it from the caliper body with a pair of pliers.


To my surprise, the piston was in fact in very good condition with no rust build up on it at all.


The threaded insert was also in place and secure, inside the piston.

I removed the piston dust seal. My thoughts now being purely that the fact that it was displaced was enough to prevent the piston retracting. I refitted the piston without the dust seal but still found that it wouldn’t retract without assistance.


I hooked out the piston fluid seal, expecting to find rust build up behind it, causing it to be too tight around the piston.


There’s plenty more information on this topic, this is just a teaser. The rest is contained within the September 2014 edition of Soft Top Hardtop, which if you’re a member you can read HERE. If you’re not a member then please join the club for more great technical help.


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