The Cessna 120-140 Association Forum

I have a 1946 Cessna 140, S/N 11581, with Scott Model 4408B4 brake master cylinders. I tried bleeding the brakes recently using the bottom-up method, because that’s much easier done without a helper, and the left brake was completed without a problem. On the right brake, though, I was able to get fluid up and into the master cylinder reservoir, but it required significantly more pressure than the left side required. For the bleeding operation, I had replaced the reservoir screw cap with a 1/4” bleeder nipple, and attached clear tubing to it, leading to a container. This was the same setup that I’d used for the left master cylinder. I wasn’t able, though, to get any fluid to exit the master cylinder reservoir in that tubing, or out of the small weep hole in the reservoir cap. I used a hand-pump hydraulic service can, and had to apply so much pressure, at that point, that the tubing would not stay on the bleeder nipple at the slave cylinder, despite several tries. There are no blockages in the brake lines or connections between the bleeder screw at the slave caliper and the master cylinder. Any ideas on what the problem might be?

It reads like the problem is with the bleeder screw. Could it be it's not open enough? Does fluid leak out with it open?

I had a similar problem and it was not open enough.

Nick

I'm not familiar with that particular master cylinder, but, the general theme is that if the pedal is not fully "up" or the internal mechanism is not all the way "released" then the passage through the master cylinder to the reservoir will be blocked. One test you can do is to loosen the fitting where the hose connects to the master and ensure you can at least pump fluid from the brake up to the master.

Thanks for your inputs. I've already disconnected the brake line connection at the master cylinder and was able to pump fluid up to that point. I also replaced the bleeder screw at the slave caliper with one with two outlet ports, in case the restriction was due to the single output port on the original bleed screw. No effect. I'm going to remove the left master cylinder, which is the same Scott model, to see if there is any difference in them, internally.

When the pedal is in the fully released position, the pushrod of the master must be all the way out against its internal stop. Only then is fluid free to pass through the master into the reservoir. Try disconnecting the pushrod from the piston. You have to correct this or the brake(s) can lock on.