Brake master cylinder not pumping ......... Why ?

[Sprocket]

The latest problem; the brake master cylinder, the one I had the window on the side broke through, but I had a spare, so I stripped it, cleaned it, checked the rubber seals were intact and reassembled wet with brake fluid and fitted to the bike along with a new brake hose.

And so there I was pumping for ages with the calliper bleed nipple open waiting for fluid to emerge, but nothing came out. So on investigation I removed the banjo bolt connecting the hose to the master cylinder and pumped the handle and no fluid came out, so all that pumping revealed the master cylinder isn't even pumping the fluid.

I cannot see what is wrong with the master cylinder, nothing is missing and everything is intact, and there is no wear or scoring on either piston rod or master cylinder pump bore, but it won't pump brake fluid - why ?


But a thought, in the master cylinder reservoir there is one hole at the base surrounded by a low wall, in front of that hole there is a dimple in the casting, should that dimple in fact be a hole and the fact that it is a dimple is because the hole is blocked as there was a degree of white corrosion in the reservoir from where the spare had been stored ?

With fluid in the reservoir and pumping, I get the occasional small bubble up from the hole with the wall around but nothing major, nothing like air is getting appreciably drawn in from anywhere, but no fluid exits the master cylinder on the way to where the brake hose would be connected.

Or can anyone give me any idea what is wrong with my master cylinder ?

The bike should have been finished this past week end, but the plastic window breaking on the master cylinder was an unforeseen problem, creating more work, but once the front hydraulic brake is fixed, that's it the bike is done- finished and usable again.

[spencer2004]

I came across a similar thing after I rebuild my calliper. You have to bleed the air out as you would bleed the fluid out; hold brake in, open bleeder valve, close bleeder valve, release brake, repeat. Brake fluid should start coming through eventually.

[998S]

Flushing a totally empty system can be a PITA.
The pump does just not catch, and it seems you are pumping air for ages.

You can do 2 things though:

1. Flush it reverse; fill a syring and a hose with brake fluid, open the valve on the calliper, and slowly fill the system from the bottom up. Just be sure your reservoir doesn't overflow.

2. Take your time, and let gravity lower the oil from the reservoir. There is a point that the system is kind of full, and the pump starts to catch. From there you can flush as u intended to do.

Or you buy special flushing pump. I never had the urge to do so though.


E.