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Saturday, 15 February 2014

A bit of a random one...Formula RX caliper rebuild...

Making bikes is ace, but riding them is even better. I realised last weekend that I had certainly let my maintenance slip on the element when I found a seized piston in the rear brake calliper and couldn't go for a ride.

As such I decided to give them a good strip down, release the seized piston and replace all the seals.

The only issue here was that I really struggled to find any literature on removing the pistons and replacing the seals on the formula RX brake callipers - so where better to put my rebuild procedure (to hopefully help out someone else trying to do the same bit of maintenance) than on my blog!

To start with I removed the callipers from my bike to wash all the crud off them - if there is one thing I've learnt about hydraulics its that cleanliness is of paramount importance.

This turned out to be fail number 1!! In hindsight I think removing the pistons would have been easier using the hydraulic force from the brakes themselves while on the bike but hey ho.

On inspection of the callipers the first thing that struck me was the large anodised red cap on the left hand side - see pic below.

I figured that you must have to remove this to service the callipers. Here began my search for the tool to remove the aluminium blanking plug. Unfortunately I couldn't find one, so in true DMO style - I just made one.

4 Pin plug extraction tool
As it turns out, and having done some more reading, you don't actually have to remove this plug to service the callipers - the hole which the plug blanks is simply there to enable the caliper to be machined - cashback!

Anywho, seeing as I had made the tool I thought I might as well use it, and as it goes it worked a treat.

Tool in position ready to remove blanking plug

Blanking plug removed - job done!
My top tip here is that, in order to undo the blanking plug, you obviously need to hold the calliper firmly. To do this I simply bolted it back onto the bike. The caps were both tight but they did undo (right hand thread) without any excessive force.

So next up was to remove the pistons - this is when I realised that the easiest way to remove the pistons is to remove the one on the opposite side of the calliper to the blanking plug first!! So back in went the blanking plug.

To remove the right hand piston simply tie a tie-wrap around the left hand piston (this stops this one coming out first) and then remove the small bleed grub screw. You need to make sure that you now blank off the "pressure in" hole on the left hand side of the calliper (where the brake hose connects to) before applying some pressure to the bleed hole.

The way I did this was to just use one of the formula bleeding syringes. The piston will just pop out. If it doesn't pop out then you will require more pressure, a compressor or even a track pump may be required depending on how corroded the pistons are in the seals. It turns out that the hose off the formula bleeding syringe is a perfect fit on my air gun - see below:

If compressed air doesn't work then the track pump should do it. In order to hook my track pump upto the formula bleed hose I found one of the removable valve inserts from an inner tube was a perfect adapter. Even with the full pressure my track pump could provide it took a fair bit of back and forth and GT85 to get the stuck piston out - but out it did come!

Next snip the tie-wrap on the left hand piston and remove the blanking plug (for the second time in my case). I then just used a 5/8's socket and my vice to press out the left hand piston, see below:

Once the pistons have been removed you can simply pick out the two piston seals and the o-ring (I used a scalpel to do this).

Next up is to thoroughly clean the calliper. I just used regular brake cleaner (Halfords, about a fiver). Once de-greased and clean I would highly recommend blowing out each small oil hole with compressed air. I was very surprised at the amount of crud that came out when I did this.

Now all the old bits have been removed and everything is clean you need to put in the right hand piston seal. Before inserting this in the calliper grease it all over with a silicon grease. 

Once the right hand piston seal is seated in its groove you need to insert the right hand piston - I used the 5/8's socket again and just pressed it into place.

Now you can grease and insert the left hand piston seal and the blanking cap o-ring.

Before inserting the left hand piston I replaced the blanking plug (just putting a touch of 243 thread lock on the thread beforehand).

To insert the left hand piston I did this by hand from the inside of the calliper body (where the brake disc runs). To seat the piston in the housing I just just used the handle of my scalpel (blade removed).

And thats about it, all that remained was to put the calipers back on the bike and bleed.

So, not much frame building here, but at least I can go out and blow off some steam on my bike!


  1. I like that; "I couldn't find one, so... I just made one". I had a pair of formula brakes and wondered what the shiny red bit did. Thanks for clearing that up. And I now know where to go to get a tool if ever I need one!

  2. Thanks for sharing. I kind of knew this would be the way to to do it by looking at the exploded diagram, but its always nice to see that someone has gone before you and not buggered it up.

    I was going to use some standard silicone oring grease, you didn't use the special avid/sram 'DOT grease' did you?

  3. Hi there may ask for suggestion
    I have difficulty to find the oring kit for this piston in my Place.. Indonesia..
    can i use some normal oring for replacing the piston oring


  4. Thank you for the instruction!