Master Cylinder Follies

OK, so I installed a shiny new steel braided line on the back of our WRX. We pumped and sucked and sucked and pumped and finally got oil coming through the line. I have bled it till I went blue in the face and there is not a bit of air in the system anywhere.

Yet there is absolutely no pressure in the system when the pedal is pushed down.

I can open the bleed screw and pump fluid through no problem but when the bleed screw is closed there is no pressure, NONE!

The only thing I can think is that we buggered up the master cylinder somehow. A seal came loose or got dislodged or torn? Something happened.

Anyway, anyone know how to tear down the rear MC on the WR and get it working again?

I have changed out brake lines maybe a dozen times before and never encountered anything like this before.

Normally when that happens it means the master cylinder needs bleeding, but the rear master cylinder on the WR doesn't have a bleeder. Overall it doesn't even look that serviceable. Not sure what to recommend other than pulling it off and tearing into it to see what you find. The service manual likely has a blow-apart of it which may help.

I thought there might be some air in the MC so I unbolted it and hung it down at ground level all night to let any air work its way up the line. There was not so much as a tiny bubble the next morning.

here is an idea if you haven't already started taking it apart again.
open your bleeder again and fully collapse the caliper piston...close the bleeder....make sure the m/c resevoir is full...then do the regular bleeding again.
the idea is there might still be some air in the caliper bore.

Have you tried reverse bleeding? It's a little more time consuming, but it'll force any air up and out of the MC and caliper.

Quote :

open your bleeder again and fully collapse the caliper piston...close the bleeder....make sure the m/c resevoir is full...then do the regular bleeding again.
the idea is there might still be some air in the caliper bore.

I already did that about 500 times. I hung the MC at ground level overnight and then bled it from ground level the next morning and not a single bubble.

Quote :

Have you tried reverse bleeding? It's a little more time consuming, but it'll force any air up and out of the MC and caliper.

I've been into bikes for more than 20 years and have never heard of reverse bleeding. Under no circumstances am I putting my lips to the bleed nipple. As much as I'd like to put my lips on a nipple ...

CDNSXV wrote:

Quote :

open your bleeder again and fully collapse the caliper piston...close the bleeder....make sure the m/c resevoir is full...then do the regular bleeding again.
the idea is there might still be some air in the caliper bore.

I already did that about 500 times. I hung the MC at ground level overnight and then bled it from ground level the next morning and not a single bubble.

Quote :

Have you tried reverse bleeding? It's a little more time consuming, but it'll force any air up and out of the MC and caliper.

I've been into bikes for more than 20 years and have never heard of reverse bleeding. Under no circumstances am I putting my lips to the bleed nipple. As much as I'd like to put my lips on a nipple ...

Most automotive bleeders are pressure bleeders, not vacuum bleeders. You can usually rig something up to the reservior cap to get fluid moving through top to bottom but the best method is bottom to top through the bleeder.

I have a big syringe I can attach to the bleed nipple. Fill that with fluid and push and I should get the same effect. I'll try that before I tear into the MC.

That's what I was referring to... I have a bottle with a length of surgical hose so the old fluid runs into the bottle and not all over everything, and a squeeze bottle with another smaller hose that I squeeze fluid from the bottom up. That's how I bleed the hydraulic brakes on my DH bike.

By the looks of it I would be happy with no front brakes!

Actually DONT open the bleeder when you push the piston back into the caliper
Master cylinder high and caliper low, push all the fluid back to the master and chances are the air will go too
I always
Bleed the master before you put the line on (plug the hole with your finger and pump)
Install and bleed the line (just pinch the caliper end of the fitting in your fingers and bleed fluid through, just let the fluid push past your fingers)
Attach the line to the caliper and bleed the caliper with the piston pushed all the way in
It's quick and just about foolproof

I betcha there's nothing wrong with the master, I wouldn't tear it apart

I tried the reverse bleed and it seemed to work. I didn't see any bubbles or anything strange. Just the fluid rose in the reservouir. Then I bled as normal and it all seemed to work properly. Still don't get why this made a difference but the brakes work now.

Very interesting. I'll have to keep that one in mind.
Maybe that will solve the brakes on the Ducati. They have 'hand of G0d' stopping power, but feel like crap right now and I haven't been able to find any air in the system either.