Inside a knockoff Brembo radial master cylinder

I got myself one of the cheap knockoff Brembo radial master cylinders that are coming out of Thailand and going for around AU$60 delivered on ebay. Build quality seems a bit iffy – the handlebar clamp surface isn’t square, and it wasn’t releasing pressure, leading to a seized brake system (not fun!). Since it didn’t cost me much (less than just a Brembo lever!), I wasn’t too upset that it didn’t work. I pulled it apart to see if I could fix it, or at least get an understanding of how they work.

Master cylinder with lever removed

Piston pushrod and cover - unscrews

Piston, pushrod and cover removed

View into reservoir port - intake/return port, equalisation port

I’m still not 100% certain on the cause of the issue. All the seals appeared ok, and no passages were blocked. I haven’t yet reinstalled and tested. Going by this thread (in particular post 5), I’m pretty sure that the cause of the issue is the piston not retracting fully so that the intake/return port was not uncovered. This would prevent pressure build up from being released. When I first installed the lever, bleeding took ages – no matter how much I pumped the lever the fluid just didn’t seem to be getting through. I had to use a syringe to force fluid up from the caliper out the bleed nipple on the master cylinder. This kind of makes sense if the intake/return port was blocked or not uncovered. Two possible causes for this could be:

  • Not enough lever free play preventing the piston from retracting enough to uncover the intake/return port, or
  • Construction tolerances out slightly so that piston does not retract far enough to uncover the intake/return port.

It is hard to tell because there’s little light, but it does seem that with the pushrod screwed all the way in, the lip of the first seal is just covering the intake/return port. I’m planning on trying it out as is, in case the cleaning did make a difference. If it still seizes, then I’m going to see about backing off the pushrod cover collar, which should allow the piston to retract slightly more. If this does work, then I’m going to need to make a spacing collar or some such.

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