The International Cessna^® 170 Association

Prior to each flight I do a two (2) step check for brake functionally. KNOW before you GO!

1. While still in hanger I have Co Pilot alternately apply brake pressure on designated brake.
I am standing over the wheel - grasp tire Rotate L to R without brake applied - Call for Brake application - can no longer move wheel - Clamped by brake Pads. Visually witness brake disk gripped by Pads.

Move out of hanger fire up engine.

TEST # 2 engine running smooth start. 1150 - 1200 RPM - PIC both brakes applied. Holding - every thing is Fine -

Suddenly Right pedal plummets down - A/C begins slow turn to the left 90 degrees-

Shut it down - Lock A/c in hanger, Again

Seek advice from forum C170.org

What do you think happened?

Shudder to think . What if i had become airborne?

I've had a brake go flat to the firewall on landing...more than once.
And the next time I used them, they were fine. Checked fluid each time and topped off but never really low.

I finally decided to rebuild the master cylinders. My guess was the lock-o-seal was failing.
After the rebuild, I never had another problem.

The lock-o-seal can become intermittent. When it does you can have perfectly performing brakes in the hanger one pump and next time you push them they will be really soft. I flew my first 170 for years this way till I figured it out. Always prepared to do a double pump on the left brake when I applied it.

Sorry to resurrect an old thread.

Bruce,

I was reading your comment and it sounded a little familiar.

I was experiencing diminishing right brake performance until there was nothing left. Had fluid topped off (from bottom to avoid getting air in the lines). After topping brakes seemed to be back to normal but once again the same problem happened. Had both master cylinders rebuilt. Brakes once again seemed back to normal.

Then on about my 4th landing of the day on my first flight after rebuild, I experienced what you described. Brake all the way to the stops and I had nothing. A quick double pump brought them back. It seems like the right brake always needs to be frequently pumped to get them working again.

Would you expect intermittent lock-o-seal after a newly rebuilt master cylinder? Did you ever figure out a permanent fix?

Thanks,
Ben

Yes Benny, this is a malfunctioning lock-o-seal. Either the seal is bad or the brake plunger is adjusted/worn so that the lock-o-seal does not consistently work.

As far as a malfunctioning lock-o-seal after rebuild. Are you sure the rebuilder knows what a lock-o-seal is and its function. I've run into A&P mechanics who did not and I had the privilege of removing the "overhauled" master cylinder and teaching them what they didn't know.

There is no permanent fix. the lock-o-seal is part of a necessary one way valve. They wear out. The permanent fix is understanding how the master cylinder works and what the components do to that end.

To re-resurrect this thread, I had the exact same problem with the intermittent brake catching and lo and behold the lock-o-seal had infact failed with the interior o-ring becoming dislodged from the outer one.

Purchased a new lock-o-seal, replaced it, careful to maintain the exact same nut location for the spring, and now the brake works every time on the initial press.

However, it will now on 1 of 10 uses slowly bleed pressure. A pump of the break instantly remedies this but this occasional slip of brake pressure if not corrected will continue until all brake pressure is lost on that side.

I tightened the nut controlling spring pressure to the lock-o-seal and on taxi the problem is still replicable.

My question is whether this is likely a problem with that nut placement allowing occasional back flow of brake fluid if the lock-o-seal slides down the cylinder over time? That's what make the most sense to me but I figure I'd pass the question to more knowledgeable minds as well. Unfortunately my mechanic is not familiar with this lock-o-seal system.

I had this image saved for a different reason, but I’ll share it here again. It is from the 100 series service manual. It has the dimension for adjusting the nut (0.040” with spring compressed).

Thank you for that image, I'll readjust again and see if that finally fixes it. It's been a stubbornly persistent issue.