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I have a bike setup with Promax Render R disc brake calipers. They use mechanical action to brake (no fluid). There's an active piston that's actuated by the brake lever, and a fixed piston that sits passively. When braking the fixed piston catches the disc and you get brake pad contact on both sides of the disc.

Anyway, I have a new set of pads that I'm trying to replace, but the pad is too large to fit in the "fixed" piston housing. Previously I was able to adjust the piston outward using a hex key or manually turning the plastic cap pictured, and then increase the surface as the pad wore. However, I can't seem to back it off any more.

What happened? Is it possible the piston has popped out of the housing? Can this be fixed?

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If the moving arm side is pre-actuated much at all (either from the barrel adjuster or being anchored that way), you may have trouble getting new pads in.

The usual procedure would be with the fixed side all the way out, put the fixed side pad in first, then the moving side. Doing it the other way might not give you enough space to get the second pad in. With those brakes and most others that use BB5 pads, the usual plan is insert the fixed pad with a needlenose plier on the tang, then shove it all the way over to one side, then do the moving pad, then put the spring in from the top as the last step.

If that doesn't address the issue, and you feel the fixed side isn't as retracted as it should be, you can take the brake apart by using a small screwdriver to gently lift off the plastic adjuster knob from the lip it hugs around. Sometimes the piston threads do get messed up on brakes like this and one can fix things with a thread file, pick, etc. It's not a common problem. There may be some kind spring and detent type thing in there, but if memory serves the piston doesn't move in notched increments on this brake, and it's just held with some loctite for extra security in the caliper threads.

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New pads are thicker than old pads because they're not worn. So you need to back out the moving piston to the maximum, and you also have to slack off the caliper mount bolts to allow the fixed pad some space. Remember to re-tighten these bolts once you have the best pad position you can get.

Remember the fixed pad wears over time just like the moving pad.

It is possible that even with piston and fixed bad stop as far apart as possible, the pads may still rub on the rotor. So you do the best you can, and then go for a short test ride to bed-in the pads. You may feel a lot of drag initially, but this should reduce as the pads bed in.

Bedding in the pads will slightly compress them, and will remove any slightly fluffy edge to the braking material. After a 10-15 minute ride with a bunch of hard stops, you should be able to spin the wheel without any brake rub. Only then can you tune the final pad position.

If you can't get a rideable initial position, try manipulating the actuator arm by hand, while looking in where the pad goes. It should move back . If not, try using your third hand to push the moving surface with something plastic like a tyre lever.

Another thought, are your new brake pads the right ones? They may be too thick ?

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