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.

Blowing lightbulbs on the XT6 – electrical problem?

Lately I’ve gone through a few headlight bulbs on the XT6 – the low beam (which is permanently on) filament has been blowing, but highbeam is ok. I thought it might be a dodgy reg/rect over-volting things, especially since my battery recently died. On checking, the reg/rect is putting out around 15.5V at idle and at 4k RPM. I also checked the output of the stator by disconnecting the reg/rect and measuring the three phase wires (white) coming into the reg/rect. At idle I was getting around 25VAC, and at 3-4k RPM about 60VAC. I’m not sure about the low voltage output at idle, but 60VAC is a good sign at 3-4k RPM.

Reading various troubleshooting guides, the general consensus is that around 14.5V out of the reg/rect is health, but anything above 15V is a worry. I tested my SRX6 and it sat nicely on 14.5V. The reg/rect it uses looks different so I didn’t want to just swap it to try.

On the suggestion of a mate, I tested DC voltage under heavy electrical load – high beam, brake light and indicator – bingo! 14.0V – 14.5V between 3k and 5k RPM. My previous testing had all been done without the headlight running (blown low beam), meaning the reg/rect had hardly any load at all. My guess is that the low load gave something almost like an open circuit voltage which is a bit higher than ideal. With a decent load (80W or so of lights) it brought the voltage down to the normal level.

Above mate also suggested that the blown lightbulbs could be to do with insufficient vibration dampening of the headlight assembly. I will have to look into improving this and see if it makes a difference.

So, I guess the morals of this story are:

  • check reg/rect output voltage under load conditions (highbeam, brake light, indicators etc) at around 5k RPM and see what you get.
  • to check coils, rather than measuring resistance, measure open circuit voltage between phases. Resistance will vary depending on power output, whereas voltage range is a functional measure. around 60VAC is good.

Here’s a good electrical troubleshooting guide http://www.offwidth.co.uk/bike/general/electrical_fault_finding.pdf